Hello,
Currently, it’s not possible from the Administration panel.
But as a workaround, I’ll share the method I use.
First of all, field sizes are stored in db inside core_store under following key:
plugin_content_manager_configuration_content_types::application::{contentTypeName}.{contentTypeName}
If you look at its value you will find field sizes stored in layouts.edit:
So basically, you can modify its value from 6 to any other bootstrap number.
If this feature is crucial to you, you could build a custom plugin that allows you to modify these values. Also, you can add that functionality that allows you to modify these values directly inside strapi-plugin-content-manager.
Imagination is the limit.
Result (col-12 / full width):
Result (col-3 / 25% of the width):
These configurations are overwritten with Strapi’s default configuration when you make changes to the content-type structure (when you create/update/delete model fields).