A person's mattress preference can be as different as a fingerprint. Mattress companies design hundreds of different models, trying to give you the right fit.
It is important to lay on the model you intend to purchase. Start by laying on your most used position. Try other positions just to be sure. Your feet should not hang over. Your elbows and knees should not touch your partner.
Something to consider when choosing the size of your bed is this: do you need room for children, grand children, and/or pets? A surprising number of customers chose a King for this reason.