When it comes to the size of your mattress, it comes down to three main considerations: the amount of space you want, the number of sleepers, and your budget. You also want to ensure your mattress will fit in your bedroom, so it’s important to first measure your room. Generally, a twin or twin extra-long mattress is big enough for one adult or child. A full-size mattress is the smallest size for two sleepers. A queen size is the most popular choice for a couple, and a king size is great for a couple that wants a bit more space. A California king is longer than a standard king but a bit narrower.