This depends on the number of people you feel may need to use it. If you and your partner enjoy spending time at the beach, then one of the smaller-sized mats will be more than enough space, unless you like bringing along a cooler, radio, beach towels, and other stuff that you want to keep sand-free.
If you have a large family, you need a mat that will be closer to the 10-ft range, or you may have to buy two mats.