Furniture dimensions don't come out of the air. They come out of heavy research that, thankfully, a lot of people have already done for us and written books on; if you're a designer, you ought have a copy of one of these books (see bottom of this entry). While these reference bibles of human dimensions haven't yet been updated to account for us supersized Americans, they still provide a good jumping-off point for determining rough dimensions, angles and heights.
Reference: Common Dimensions, Angles and Heights for Seating Designers