I think this is totally a designer decision. When you allow your user 10 MB file sizes the environment file size limit will be reached sooner. It depends completely on the sort of documents. It is simply something you have to discuss with the client if documents needs to be stored in the system. If the documents are needed in the application the client needs to purchase additional diskspace.
The other option is to tranfer the created document to another cloud based entity like dropbox or Onedrive (the last one gives 1 TB of diskspace with a 365 plan) and store only the links in Mendix.
I usaly leave the max file size to the standard of 5 MB unless the customer wants otherwise.