In the United States, the mattress industry has adopted standard sizes. Sheetings, bedspreads, and mattress pads should be purchased to match the size of the mattress, and they are labeled to indicate the mattress size they fit.
For interior design, the dimensions of mattresses may be useful in
The King bed is substantially wider than the Queen at 76”. That’s over 6 feet wide. Like the Queen it is 80” long. The California King is slightly narrower (72”) but even longer at 84”. Sheets for King-sized beds are more expensive and they require three pillows.
The Salvation Army will take donations of mattresses, but they prefer items that are in decent condition. Don't call them if your old mattress is too dilapidated. The good thing about the Salvation Army is that they will come pick up the mattress, and sometimes you can even leave the mattress outside if you aren't going to be home. Call your local Salvation Army (they are in the phone book) to confirm that they will take mattresses and to schedule a pick up.
If the mattress is too sunken or too torn up or you've been letting your dogs sleep on it, don't give it to charity. Throw it away. Two possibilities here: (1) take it to the dump/tip or (2) have your garbage collection service pick it up.
If you have a truck or vehicle big enough for the mattress, you can load it up and drive to the landfill. They will probably charge you a tipping fee. Alternatively, there are companies that will carry off large items like this for you, for a fee. Look in the yellow pages or do an internet search for junk or recycling or waste disposal.
Your municipal garbage service will probably take the mattress, but not as a part of their routine service. Some services specify certain days or weeks during the year when they will do "bulk pickup". At those times you can put your old mattress out on the curb and a special truck will come by and haul it away. On most normal pickup days, most services will not take away your mattress. It is difficult for us to give general guidelines on this matter. Call your local garbage people - in the phone book under county or city solid waste services.
Conigliaro Industries's website says recycling a mattress saves 23 cubic feet of landfill space. They are promoting their recycling system, so you have to take that into consideration
Slate magazine has an article on "how to be eco-friendly when buying a mattress" which says the carbon footprint of a mattress is negligible, but the manufacturers use green pitches to sell their product. The industry has even established a "certified" foam mattress program.