Choosing a mattress size depends on several factors:

• Number of people/pets - If you have a partner, pets, or children, you're going to need more space.
• Room size - You may want a large mattress, but it's a good idea to know the dimensions of your room to have room for a side table or two, room for your bed frame, and for clearance to walk around the bed.
• Height and weight - Small mattresses can be uncomfortable for large people. Make sure you have at least a little space above your head and below your feet, and enough room on your sides so you don't feel cramped.
• Movement - If you move a lot, especially if you flail your arms and extend your legs to the sides, you'll want extra space. Even more so if you sleep with a partner.

Other factors to consider when choosing a mattress:

• Budget - Larger mattresses are more expensive.
• Softness/Support - Side sleepers often need more support compared to back and stomach sleepers. Softer mattresses tend to reduce motion transfer.

Coolness - It is recommended that you sleep in a cooler environment for a more restful sleep. Some mattresses are better than others at not retaining heat and then reflecting your body heat back onto you.

When deciding what size mattress to buy, there are a few things to keep in mind other than those listed above. Smaller mattresses are easier to move. The average life of a mattress is around 8 years, so if you know you're going to be moving within that time, understand that larger mattresses are more difficult to move, especially if they're foam mattresses. Your children will grow. For example, if you and your spouse are tall, and you buy your 11-year-old a twin or a double mattress, they may outgrow their mattress when they hit puberty.

Mattresses vary in thickness from 6" to 18". The thicker mattresses are more expensive and often offer more support because they have more and thicker layers. Most mattresses have between 2 and 5 layers.

The top 1-2 layers are usually the comfort layer to make the mattress softer. These may include a cover and a pillow top made of foam or a foam/gel combination. These layers are responsible for a feeling of softness, but also play a big role in motion transfer and heat transfer. The thicker these layers are, the softer the mattress will feel. And for larger people, a thicker comfort layer is usually recommended, as their weight will compress the foam of a thinner mattress until they feel the firmer support layer below, which may be uncomfortable. These top layers should be at least 2" thick, though many sleepers prefer an even thicker top.

The next layers are support layers that help keep your spine in alignment. If a mattress only had a comfort layer, the soft material would compress and you would feel the hard surface of the surface below, which would be uncomfortable. Most mattresses sit on a box spring, slatted base, or platform. The support layers compress and prevent you from feeling the surface that your mattress is sitting on, and are usually at least 4" thick. If you're placing your mattress directly on the floor, it's recommended to have a thickness of at least 12", as the floor won't give at all, as opposed to a box spring which will compress.

Thicker mattresses, especially in the foundation layer, may last longer because they keep their shape. This is why thicker mattresses often have longer warranties. If the foundation layer bends, the mattress will sag, usually in the middle or in the middle of your side of the bed, creating valleys in your mattress that you fall into and can be hard to pull yourself out of. These sagging areas in a mattress are a sign that you need to buy a new mattress.

Side sleepers also tend to need a thicker mattress because their weight is distributed mainly in their hips and shoulders, as opposed to a back or stomach sleeper that more evenly distributes their weight across their entire body. If you put your hand on a mattress and are able to put all of your weight on that one hand, you will compress the mattress more because all of your weight is in one spot. When you lie all of your body on a mattress, the weight is spread out and doesn't compress the mattress as much.

A thicker mattress will have less motion transfer than a thinner mattress of the same material.

A drawback of thicker foam mattresses is that they tend to get hot during the night. In a box spring mattress, the heat dissipates into the large empty spaces in the mattresses. In cheaper foam mattresses, the heat has nowhere to go and gets radiated back to the sleeper. Higher quality mattresses reduce or prevent this by adding gel to the foam, using an egg crate design or using an open-cell foam.

When buying bedding for your mattress, make sure that it will fit your mattress. If you have a sheet that's too thick, it may bunch up and slip. If it's too thin, it won't hold on your mattress. Look for a depth of 2" deeper than your mattress width.

A drawback of a thicker mattress is that it can make it harder to get into bed. If you have any mobility issues, you don't want to have to jump to get yourself into bed. If this applies to you, measure the height of everything you have below your mattress to see what size mattress you need. In total, you probably won't want a total of more than 22", which is the height of most chairs.

If you need a thicker mattress, you may be able to buy a smaller or shorter foundation for your mattress, depending on the size of your current setup.

Keep in mind that buying a less common mattress size may present some problems. Finding bedding will be more difficult. If you're walking through a retail store and find a comforter that you like, it won't come in a Wyoming or Alaska King size. You'll probably also need a custom-made bed frame, which will be more expensive and may take a few weeks to months to build, stain, and ship.