Yeah, it's a really common complaint, but definitely a very simple and doable process (easier than getting the image on, in fact)

A4 works fine. I normally try to get a screen a bit bigger, as it makes printing easier, so A3 or so. The one I normally use is 50 x 40cm (from memory), but that just means I can get a few designs on and need a good support/bracket system when printing.

The most important thing is the mesh. Honestly, can't stress it enough. Bad mesh= bad prints, every time. You can get away with an awful frame, but the mesh makes the image. A good 70T yellow mesh will do almost anything you'd need, but if you want small details (small font, or halftones) I'd use 100T like this:

https://gjs.co/supplies/p3371/100t-screen-mesh