Rendering can be any size, so basically whatever is needed is what is rendered.
Also, I'm not sure if you understand or not, but FFVII's maps were all high resolution models that were rendered in the first place. We are recreating those models within a reasonable tolerance of error, and rendering them at probably somewhere around 4x to 8x the original size so they don't look pixelated in-game.
As for a list of files... I don't think ANY scenes have been finished yet, and really there's no telling what can happen when many people work on one scene, you could get 3 steaming piles of crap and one absolutely amazing piece, or you could get all amazing pieces in which you are conflicted in your choices... or you could get all crap...
Many artists are better than one in my opinion.
If you want to help, pick your favorite place and have at it.
As for the supposed "sprites" for lighting... we'll do exactly what square did and cut away everything in the render except for the pixels we want to be in the foreground. Square made their layers after the fact, that's why the antialiasing is still present in scenes... Sometimes if you look for it, you can see the AA in front of the characters.
FFVII doesn't do variable or semi-transparency at all, so we simply have "opaque" or "transparent" pixels to worry about.
The only real issue is getting the scenes rendered at the same FOV and angle as the originals.