Re: What do I set my paper size to??
Jumping in, hiya doin!....
I don't think in terms of paper size unless my intentions is to end up in print.
My thinking even then is generally in pixels and then to paint as large as my system allows with good function. My imac can handle Photoshop files in layers easily at 5000 x 7000 pixels, 8000 x 10000 a little slower.
To my way of thinking it's more important to decided on height to width ratio first, just as you would with in a pigment painting. You know, does a scene look better square, landscape, portrait, cinematic? Then work out the pixel size and worry about the print resolution last, if at all.