Ok... I'm planning on adjusting a chair pattern to hold a large guy... As in 20 inch seat height. 24 inch wide seat. Would it work to use bigger pieces of wood for the supports or should I double up on the supports? Would it look terrible next to a normal size of the same chair? How to I go about making the weight limit much higher?
It will look more proportionally correct if you use bigger pieces of wood. The bigger pieces will be inherently stronger, but pay attention to the joints, as those will need to be stronger too. I'm guessing the joints will be at greater risk of failure than the thicker wood.
Define "large". Then we can do some engineering calca on the span and weight load.
As the previous poster said, the joints are going to be your weak point. Make sure that your weight is being held up by a platform on a column of wood (seat over chair legs), not a platform connected to the column of wood by screws (seat hung between chair legs).
What about appropriate sized screws or bolts? I have a friend who asked me to help her build an adirondack chair for her and her husband and they are each 300 pounds plus.