This is what I do for place mats, and yes, I birth them too.
I make any 12 inch block I like. Then I add a one -two inch border all around the block. Then I add a pieced border on the outside on the sides of flying geese or something eye catching. The place mat ends up being (with 1 inch top/bottom and 1 +2 on the sides) 13 1/2 by 17, the size of most place mats.
I sometimes use the same fabric or color for the 1 inch border for all 4 to coordinate.