Hi-hi! Glad you like the look of things so far!
I never did optimization testing so I don't actually have answers for those questions strictly speaking. The more items you add, the more resources the program will use. I once saw someone add in all the Pokemon pixel sprites but it was really laggy during use. So my general guess is to try and keep the number lower and just pay attention to how the program functions with other things running.
You can add images of any size and they'll be scaled down, but I suggest trying to keep things close to the size you'll want them on stream. So usually in the 200 px or less range.