What pan sizes are used for a 1/4, 1/2, 3/4 and whole? What kind of prices do you charge and is it for one or two layers?
Those are two threads with some information about sheet cakes and sizing.
I don't use the 1/4, 1/2 or full sheet terms myself. I go by how many servings are needed, because if they are only feeding 20 people they really don't need a full sheet cake.
I use the above chart for sizing. If your 'sheet' cake is only 1 layer, then you would divide the servings in half. Some people only do one layer and cut 2x2x2 inch slices, other people will do 2 layers and cut 1x2x4 in pieces.
How much you charge depends on a lot of different factors. You really need to figure out how much it costs you to make them, including everything you use. How long it will take you to make them. How much your time is worth. How much you want to make in addition to those to things.