When we started building our trailer the inspector told me there really wasn't a set size that they required. He said be real and put in decent sized sink, but he has approved some very small set ups.
Do yourself a favor and call your health dept and make friends with your inspector. They will help you out in the long run and save on some headaches as well.
KCBS CBJ 9831
Large Spicewine, Backwoods Smoker (Party), WSM, Oklahoma Joe's Offset, Wood Fired Oven