Below is a chart of standard mattress sizes commonly available in the United States. It can be difficult to tell the difference between the 54" wide Full or Double size mattress and the 60" wide Queen. Use it to measure your current mattress or frame if you are unsure which size you own.
It is a common misconception California King is the largest size of mattress. Almost no one who does not have a waterbed has a California King. It should generally not be purchased, instead buy the more common Eastern (Standard)King. Not only are sheets and wood beds easier to find and cheaper in Eastern King, but thanks to Eastern King's greater width Eastern King provides more usable sleeping space for individuals under 6'6".

Twin = 39 x 74
Full = 54 x 74
Queen = 60 x 80
King (aka Eastern King) 76 x 80
California King 72 x 84 (rare and narrow)