rptest: reduce HighThroughputTests.msg_size

[andrewhsu]

Fixes https://github.com/redpanda-data/core-internal/issues/986

Reduce to a range between 1K to 16K which is what we usually want to test within.

example test run:

ducktape \ 
 --debug \ 
 --globals=/home/ubuntu/redpanda/tests/globals.json \ 
 --cluster=ducktape.cluster.json.JsonCluster \ 
 --cluster-file=/home/ubuntu/redpanda/tests/cluster.json \  
 --test-runner-timeout=3600000 \
 tests/rptest/redpanda_cloud_tests/high_throughput_test.py::HighThroughputTest.test_throughput_simple

output:

test_id:    rptest.redpanda_cloud_tests.high_throughput_test.HighThroughputTest.test_throughput_simple
status:     PASS
run time:   1 minute 5.736 seconds
-----------------------------------------------------------------------
=====================================================
SESSION REPORT (ALL TESTS)
ducktape version: 0.8.18
session_id:       2024-01-19--016
run time:         1 minute 5.756 seconds
tests run:        1
passed:           1
flaky:            0
failed:           0
ignored:          0
opassed:          0
ofailed:          0
=====================================================

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x
• v23.1.x

Release Notes

• none