The best trees for stump chairs are at least 18 inches in diameter, so that you have enough thickness to cut the angle for the back and still have enough seat to sit on. Smaller trees can be used, but it is helpful to have an exterior plywood base under the seat to support the boards and be able to overhang enough to fit a person. The Spruce Stump Chair is a 12 inch tree with this kind of overhang. This bench has a half inch overhang on the seat, with more on the back to create a smooth round.