diff --git a/lib/rails_bootstrap_form/bootstrap_form_options.rb b/lib/rails_bootstrap_form/bootstrap_form_options.rb
index a14069b..aa2236f 100644
--- a/lib/rails_bootstrap_form/bootstrap_form_options.rb
+++ b/lib/rails_bootstrap_form/bootstrap_form_options.rb
@@ -94,6 +94,10 @@ class BootstrapFormOptions
     # Default is `{}`.
     attr_accessor :wrapper_options
 
+    # Option to specify the size of input groups and fields.
+    # The valid values are `sm` and `lg`. The default value is `nil`.
+    attr_accessor :size
+
     def initialize(options = {})
       set_defaults
       set_bootstrap_form_options(options)
@@ -154,6 +158,8 @@ def set_defaults
       @switch = false
 
       @wrapper_options = {}
+
+      @size = nil
     end
 
     private :set_defaults
diff --git a/lib/rails_bootstrap_form/field_wrapper_builder.rb b/lib/rails_bootstrap_form/field_wrapper_builder.rb
index 63a2360..ab9b37f 100644
--- a/lib/rails_bootstrap_form/field_wrapper_builder.rb
+++ b/lib/rails_bootstrap_form/field_wrapper_builder.rb
@@ -61,6 +61,9 @@ def field_css_options(attribute, bootstrap_options, options, html_options)
         bootstrap_options.additional_field_class
       ]
       field_classes << "is-invalid" if is_invalid?(attribute)
+      if is_size_valid?(bootstrap_options)
+        field_classes << "#{bootstrap_options.field_class}-#{bootstrap_options.size}"
+      end
 
       css_options[:class] = field_classes.flatten.compact
       css_options.merge!(required_field_options(attribute, options))
diff --git a/lib/rails_bootstrap_form/input_group_builder.rb b/lib/rails_bootstrap_form/input_group_builder.rb
index b9be842..2289f61 100644
--- a/lib/rails_bootstrap_form/input_group_builder.rb
+++ b/lib/rails_bootstrap_form/input_group_builder.rb
@@ -26,7 +26,10 @@ def input_group_wrapper(attribute, bootstrap_options, &block)
       end
 
       def input_group_classes(attribute, bootstrap_options)
-        classes = ["input-group", bootstrap_options.additional_input_group_class]
+        classes = Array("input-group") << bootstrap_options.additional_input_group_class
+        if is_size_valid?(bootstrap_options)
+          classes << "input-group-#{bootstrap_options.size}"
+        end
         # Require `has-validation` class if field has errors.
         classes << "has-validation" if is_invalid?(attribute)
         classes.flatten.compact
diff --git a/lib/rails_bootstrap_form/inputs/base.rb b/lib/rails_bootstrap_form/inputs/base.rb
index 5bb314e..4b27014 100644
--- a/lib/rails_bootstrap_form/inputs/base.rb
+++ b/lib/rails_bootstrap_form/inputs/base.rb
@@ -17,7 +17,11 @@ def control_specific_class(field_tag_name)
           "rails-bootstrap-forms-#{field_tag_name.to_s.tr("_", "-")}"
         end
 
-        private :collection_input_checked?, :control_specific_class
+        def is_size_valid?(bootstrap_options)
+          bootstrap_options.size && %i(sm lg).include?(bootstrap_options.size)
+        end
+
+        private :collection_input_checked?, :control_specific_class, :is_size_valid?
       end
     end
   end