Size Guide. Buying a too-small mattress is one of the biggest mistakes new bed-buyers make. Queen Mattress. Queen mattress dimensions are 60 inches wide by approximately 80 inches long 6 inches wider and 5 inches longer than a full size mattress. Don’t worry, at Dreams we stock European sizes, so just measure your bed to make sure you know exactly what size you need. US this mattress is referred to as a ‘Queen’ and is known in Japan as a ‘Wide Double’. They differ when it comes to width, however. At 54 inches (137 cm) wide, a full size bed is nearly as wide as a queen size bed.