Before buying any pad, sleep on it, literally if you can, or figuratively by carefully weighing all the factors you're looking for in a pad. Thickness, length, and width are all very subjective needs.
The cushier and larger the pad, the bulkier and heavier it is to pack, so consider your comfort threshold carefully. Do you typically camp in soft-floored pine needle forest settings or in rocky, high alpine sites? Do you sleep cold or warm? If you're a small person, or are willing to put an empty pack under your feet, consider 1/2- or 3/4-length pads to save weight.
If you do winter camping, consider getting two pads: a closed-cell foam pad for an insulating, waterproof bottom layer, and then your choice of upper layer pad. Some pads come with both layers integrated into one, usually enclosed in a fabric sleeve.