diff --git a/CHANGELOG.md b/CHANGELOG.md
index 020c9b16c7..c76f879362 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#6441](https://github.com/thanos-io/thanos/pull/6441) Compact: Compactor will set `index_stats` in `meta.json` file with max series and chunk size information.
 - [#6466](https://github.com/thanos-io/thanos/pull/6466) Mixin (Receive): add limits alerting for configuration reload and meta-monitoring.
 - [#6467](https://github.com/thanos-io/thanos/pull/6467) Mixin (Receive): add alert for tenant reaching head series limit.
+- [#6528](https://github.com/thanos-io/thanos/pull/6528) Index Cache: Add histogram metric `thanos_store_index_cache_stored_data_size_bytes` for item size.
 
 ### Fixed
 - [#6503](https://github.com/thanos-io/thanos/pull/6503) *: Change the engine behind `ContentPathReloader` to be completely independent of any filesystem concept. This effectively fixes this configuration reload when used with Kubernetes ConfigMaps, Secrets, or other volume mounts.
diff --git a/pkg/store/cache/cache.go b/pkg/store/cache/cache.go
index bcbb727b30..ad6ecac30e 100644
--- a/pkg/store/cache/cache.go
+++ b/pkg/store/cache/cache.go
@@ -58,8 +58,9 @@ type IndexCache interface {
 
 // Common metrics that should be used by all cache implementations.
 type commonMetrics struct {
-	requestTotal *prometheus.CounterVec
-	hitsTotal    *prometheus.CounterVec
+	requestTotal  *prometheus.CounterVec
+	hitsTotal     *prometheus.CounterVec
+	dataSizeBytes *prometheus.HistogramVec
 }
 
 func newCommonMetrics(reg prometheus.Registerer) *commonMetrics {
@@ -72,6 +73,13 @@ func newCommonMetrics(reg prometheus.Registerer) *commonMetrics {
 			Name: "thanos_store_index_cache_hits_total",
 			Help: "Total number of items requests to the cache that were a hit.",
 		}, []string{"item_type"}),
+		dataSizeBytes: promauto.With(reg).NewHistogramVec(prometheus.HistogramOpts{
+			Name: "thanos_store_index_cache_stored_data_size_bytes",
+			Help: "Histogram to track item data size stored in index cache",
+			Buckets: []float64{
+				32, 256, 512, 1024, 32 * 1024, 256 * 1024, 512 * 1024, 1024 * 1024, 32 * 1024 * 1024, 256 * 1024 * 1024, 512 * 1024 * 1024,
+			},
+		}, []string{"item_type"}),
 	}
 }
 
diff --git a/pkg/store/cache/inmemory.go b/pkg/store/cache/inmemory.go
index d5227285a2..747199b414 100644
--- a/pkg/store/cache/inmemory.go
+++ b/pkg/store/cache/inmemory.go
@@ -295,6 +295,7 @@ func copyToKey(l labels.Label) cacheKeyPostings {
 // StorePostings sets the postings identified by the ulid and label to the value v,
 // if the postings already exists in the cache it is not mutated.
 func (c *InMemoryIndexCache) StorePostings(blockID ulid.ULID, l labels.Label, v []byte) {
+	c.commonMetrics.dataSizeBytes.WithLabelValues(cacheTypePostings).Observe(float64(len(v)))
 	c.set(cacheTypePostings, cacheKey{block: blockID.String(), key: copyToKey(l)}, v)
 }
 
@@ -318,6 +319,7 @@ func (c *InMemoryIndexCache) FetchMultiPostings(_ context.Context, blockID ulid.
 
 // StoreExpandedPostings stores expanded postings for a set of label matchers.
 func (c *InMemoryIndexCache) StoreExpandedPostings(blockID ulid.ULID, matchers []*labels.Matcher, v []byte) {
+	c.commonMetrics.dataSizeBytes.WithLabelValues(cacheTypeExpandedPostings).Observe(float64(len(v)))
 	c.set(cacheTypeExpandedPostings, cacheKey{block: blockID.String(), key: cacheKeyExpandedPostings(labelMatchersToString(matchers))}, v)
 }
 
@@ -332,6 +334,7 @@ func (c *InMemoryIndexCache) FetchExpandedPostings(_ context.Context, blockID ul
 // StoreSeries sets the series identified by the ulid and id to the value v,
 // if the series already exists in the cache it is not mutated.
 func (c *InMemoryIndexCache) StoreSeries(blockID ulid.ULID, id storage.SeriesRef, v []byte) {
+	c.commonMetrics.dataSizeBytes.WithLabelValues(cacheTypeSeries).Observe(float64(len(v)))
 	c.set(cacheTypeSeries, cacheKey{blockID.String(), cacheKeySeries(id), ""}, v)
 }
 
diff --git a/pkg/store/cache/memcached.go b/pkg/store/cache/memcached.go
index f5ab1c4b02..9292f3ed59 100644
--- a/pkg/store/cache/memcached.go
+++ b/pkg/store/cache/memcached.go
@@ -33,12 +33,15 @@ type RemoteIndexCache struct {
 	compressionScheme string
 
 	// Metrics.
-	postingRequests         prometheus.Counter
-	seriesRequests          prometheus.Counter
-	expandedPostingRequests prometheus.Counter
-	postingHits             prometheus.Counter
-	seriesHits              prometheus.Counter
-	expandedPostingHits     prometheus.Counter
+	postingRequests              prometheus.Counter
+	seriesRequests               prometheus.Counter
+	expandedPostingRequests      prometheus.Counter
+	postingHits                  prometheus.Counter
+	seriesHits                   prometheus.Counter
+	expandedPostingHits          prometheus.Counter
+	postingDataSizeBytes         prometheus.Observer
+	expandedPostingDataSizeBytes prometheus.Observer
+	seriesDataSizeBytes          prometheus.Observer
 }
 
 // NewRemoteIndexCache makes a new RemoteIndexCache.
@@ -61,6 +64,10 @@ func NewRemoteIndexCache(logger log.Logger, cacheClient cacheutil.RemoteCacheCli
 	c.seriesHits = commonMetrics.hitsTotal.WithLabelValues(cacheTypeSeries)
 	c.expandedPostingHits = commonMetrics.hitsTotal.WithLabelValues(cacheTypeExpandedPostings)
 
+	c.postingDataSizeBytes = commonMetrics.dataSizeBytes.WithLabelValues(cacheTypePostings)
+	c.seriesDataSizeBytes = commonMetrics.dataSizeBytes.WithLabelValues(cacheTypeSeries)
+	c.expandedPostingDataSizeBytes = commonMetrics.dataSizeBytes.WithLabelValues(cacheTypeExpandedPostings)
+
 	level.Info(logger).Log("msg", "created index cache")
 
 	return c, nil
@@ -70,6 +77,7 @@ func NewRemoteIndexCache(logger log.Logger, cacheClient cacheutil.RemoteCacheCli
 // The function enqueues the request and returns immediately: the entry will be
 // asynchronously stored in the cache.
 func (c *RemoteIndexCache) StorePostings(blockID ulid.ULID, l labels.Label, v []byte) {
+	c.postingDataSizeBytes.Observe(float64(len(v)))
 	key := cacheKey{blockID.String(), cacheKeyPostings(l), c.compressionScheme}.string()
 	if err := c.memcached.SetAsync(key, v, memcachedDefaultTTL); err != nil {
 		level.Error(c.logger).Log("msg", "failed to cache postings in memcached", "err", err)
@@ -118,6 +126,7 @@ func (c *RemoteIndexCache) FetchMultiPostings(ctx context.Context, blockID ulid.
 // The function enqueues the request and returns immediately: the entry will be
 // asynchronously stored in the cache.
 func (c *RemoteIndexCache) StoreExpandedPostings(blockID ulid.ULID, keys []*labels.Matcher, v []byte) {
+	c.expandedPostingDataSizeBytes.Observe(float64(len(v)))
 	key := cacheKey{blockID.String(), cacheKeyExpandedPostings(labelMatchersToString(keys)), c.compressionScheme}.string()
 
 	if err := c.memcached.SetAsync(key, v, memcachedDefaultTTL); err != nil {
@@ -148,6 +157,7 @@ func (c *RemoteIndexCache) FetchExpandedPostings(ctx context.Context, blockID ul
 // The function enqueues the request and returns immediately: the entry will be
 // asynchronously stored in the cache.
 func (c *RemoteIndexCache) StoreSeries(blockID ulid.ULID, id storage.SeriesRef, v []byte) {
+	c.seriesDataSizeBytes.Observe(float64(len(v)))
 	key := cacheKey{blockID.String(), cacheKeySeries(id), ""}.string()
 
 	if err := c.memcached.SetAsync(key, v, memcachedDefaultTTL); err != nil {