Bones, you need to consider the ergonomics of seating. The bible for data is 'Human Dimension and Space' by Zelnek and Panero, the data is a bit dated now but is a good guide. For seat height measure a couple of dining chairs that you find comfortable, then the seat depth. This will give you your basic dimensions. Remember that the back should be at an obtuse angle to the seat (somewhere round about 95degrees) If the seat is too high you put pressure on the underside of your thighs which is not good for circulation - its why office chairs have a 'waterfall' front. On most boats unlees you put the dinette on a plinth the seat will be too low to see out of the window. Having said all that its possible to design a dinette which satisfies most ergonomic requirements using a standard sheet of ply and good quality cushions.