With the mention of how oversized beds are suitable for incredibly tall individuals, you may wonder what type of bed an average-height adult needs. Those who are roughly 6 feet tall should use a bed 80 inches or longer. Twin XL, queen, king, and California king mattresses are all suitable standard size mattresses 80 inches or longer. Small single bed sizes like twin XL mattresses are suitable for dorm rooms, bunk beds, and small homes, while the larger mattress sizes fit best in master bedrooms or guest rooms.