I know this topic hasn't been replied to in a while, but I think custom brush size is kinda redundant, since I'm not even sure how it'd be implemented.
We should have 1x1, 2x2, 4x4, 6x6, and 8x8 brushes instead of 1x1, 3x3, 5x5, 7x7, and 9x9. I never saw a use in brushes larger than 5x5, especially since there are already other tools that exist that make room editing easier