Is there a maximum number of images you can have in a premium shop? Or a maximum size of store files (i.e. megabyte limit?)?

There is no stated maximum. They reserve the right to remove files that aren't being used, although I haven't had that happen yet. There is a 500 section maximum for premium shops, but no stated limits on the number of products per section. For images you will want to break them up into manageable units in folders. I find that if I get more than three pages to a folder it gets cumbersome. I think I'm moderate in my number of images and I have a little over 3,000 images.

Thanks. I think I need to work on my folder structure as I'm ending up with a large number of pages in a few of them. I'm expecting to probably 10K+ designs in use when I complete them.