Need help on building bench seats. Ive bought the foam and vinyl,?

I just need to know if i should cut the backing board of the seat to exactly the same size as the foam or should i cut the board like a 1/4 inch under on all sides? or what the best line of attack for this would be.
you obviously have no idea what you are talking about. cutting wood it 900 times easier than cutting 4 inch foam to a desired dimension.

Make your bench seat to the size you want, then you fit the foam to them. You are going about this backwards.