For the pixel assets the weird 21x21 size is because I really wanted 1 pixel to be the center, so that all tiles have a center and look great. As said, modern machines can handle any size texture really well.
I've just created a new pack, especially made for creating boardgames. Includes 19 pieces in 7 colors and 3 styles, playing cards and dice in 2 colors. Also includes spritesheets and vector files, licensed CC Zero!
Download:
http://opengameart.org/content/boardgame-pack