Limiting User Uploads with Trix Editor

I'm adding comments to my posts, very similar to https://gorails.com/episodes/nested-comment-threads-in-rails-part-1 and using the Trix Editor.  

I want to allow user-uploads, but of course I want them to be within reason.  I don't want to pay to store someone's 100MB photos.  Any ideas on how to put limitations on the uploads?  (size and filetype limitations).

Thanks!