A: Great question, Sam! The average shoe size that fits best for the footed section of these adult onesies for men are Small (S — ~5′6″–5′8″): Shoe size ~7–9. Medium (M — ~5′8″–5′10″): Shoe size ~8–11. Large (L — ~5′9″–6′1″): Shoe size ~9–11. Extra Large (XL — ~5′9″–6′1″): Shoe size ~9–12. This is a general guide because the boot section of this men's adult onesie isn't fitted, a larger foot will pull down and use some of the extra fabric on the top of the foot. Smaller feet will then use less of the bottom section of the foot and some extra fabric will be at the top of the foot of the fleece onesie for men. Hope this helps!