Tile size will be 32x32 as 48x48 will give technical problems. The character mask will have to depend on the final sprite, but I'm not sure what that will be yet. I haven't made much progress on my template for the sprites, but I haven't deleted them all yet, so there's that.
Here's an example of what size they'll be if I finish them:

As for pixels/meter, we obviously need to base them off of whatever sprite we use in the end.