The seats designers must be careful with some basic ergonomics rules that makes the furniture compatible with the person who use it (Seating Designers Secrets: Common Dimensions, Angles and Heights). The seating chairs, sofas, armchairs, office chairs and similar must be appropriate with the height and regular human body’s dimensions.
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 supersize Americans, they still provide a good jumping-off point for determining rough dimensions, angles and heights.