For all intents and purposes - its whatever size you deem best, but there are a few somewhat standard sizes for particular shapes of tables and how many individuals can be seated. A lot of the ones I've made that are somewhat universal are 12x36" and 12x60" which seem to be a good happy medium for most people. However, I always take into account who its for and how big their table is. Bottomline, do what is most pleasing for you and compliments your particular structure, size and shape of the table you are making this for.
Explosive blessings, abundance and inspiration to you all!
- The Creative Seamstress