If I look in the charts it says a 10" square will feed 40. But for a family gathering do you think it would serve 30 OK and not be skimpy?

it will serve 30 1 1/2"x2"x4" slices

so should I add another 10" to make it 6" tall....?

A 10" square is more than enough for 30 people. Square cakes feed a lot more than rounds and a 10" round will feed 30 wedding slices so I think you will be more than fine.

Thanks, and its got 3 or 4 cupcakes going with it also...