OK, I've got the standard Gallery installed which doesn't do autoresize. We'll have to update to the Pro version for $50. We'll do that after we get the domain name in place because they tie it to the domain name. In the meantime, I've allowed pix up to 4500 px in either dimention with a max size of 5mb. We'll want to take it easy on the sizes because they will eat mobile data like crazy on uploads & downloads.
I set my phone to max resolution but the pix are automatically uploaded to Google Photos and resized. When I want to post them somewhere, I just download from photos to my computer and they are ready to go. Hmm, that gives me an idea. I can send from my phone to facebook, which resizes and I think I can share facebook albums on SMF since my accounts are linked. I'll have to try that.