Sleeping pads divide roughly into three types:
Within each type, you can choose among various lengths and thicknesses. For car camping, reviews recommend the biggest, thickest sleeping pad that will fit your sleeping bag and tent, since weight isn't usually an issue. These sleeping pads can be quite luxurious, with price the main drawback.
The lightweight three-season sleeping pad is the most popular type. Reviews say the length and thickness depend on each individual's priorities. Some backpackers find it's better to carry the extra weight of a full-length, wider and/or thicker sleeping pad than to endure sleepless nights or suffer stiffness and aches upon awakening. At the other extreme, others do fine with very short torso pads plus clothes for a pillow and a backpack under the feet. The three-quarter-length sleeping pad (about four feet long) is a compromise between these extremes. The balance between comfort and weight becomes very delicate, especially on long hiking trips. Quite a few lightweight sleeping pads come in women's versions, which provide extra insulation for the torso and feet.
For colder nights, a full-length sleeping bag is needed for insulation from the cold ground, so backpackers save weight by using narrow and thin sleeping pads – often in layers – and sometimes tapered to fit a mummy sleeping bag. For cool nights, R-values -- a standardized measure of how much a material impedes the flow of heat -- can be important in comparing sleeping pads.