Well, in mudbox texture resolution doesent look at the polys densisty, so for paint an hig res map you dont need milions of polygons, like in zbrush.
So you decide how smooth you want the model to look when it comes to paint, of course painting on an higres model is better than a low res one.
The mesh dimension SHOULDN'T matter, im not sure, just give it a try: make a big model and a small model, then subdivide them, and scale them so they are the same dimension and check the topo 🙂
