Thank you for writing to us and welcome!
In general, the files would be small enough (under 10mb each). It really depends on the type of creation - if you start using textures or the “freestyle” brush, the file might be bigger.
If you create an account in uMake, you can backup all your files to the cloud, which means that only the files you opened and edited, will be downloaded to your iPad (to save space). Additionally, you can export all your designs as .UMK (uMake file format) to iCloud, Dropbox, Google Drive, etc, or via AirDrop to your Mac and import them back to uMake when you need to.
I’m using the 11" 64GB and it’s more than enough for me. It depends on how do you use the iPad and how much of its storage goes to Photos (if you’re not saving those in the cloud) and other apps.
Hopes this answers your question.