It depends on how you plan to use the space, but I suspect that if the incremental cost isn't too high you will be happier if you add a few feet. This is particularly true if you want both dining and seating areas. Think of it as a room and draw a furniture plan. Be sure to leave aisle room between the separate areas, enough push back space for dining chairs, and a clear passageway from the door to the backyard.

Agree that if you want both dinng and seating you will need a little more space. in addition to the 3' occupied by the grill/ sink you will need about 3' aisle work space. A dining table will take up about 9' minimum and you will want at least about 8' for seating. So you're at 23'. You may get away on going a little shallower to 14', depends on where the doors are and traffic flow.

Our covered patio is 20'x 20', with and additional 8x8' cooking area bump-out. we have a small dining table , sectional sofa, and porch swing, and extra floor space for our son to play.