a good dimention (for your first one) is 20' up to 16' flat to 20' down. 20' flat on top is good too, but you'll have to go a bit faster to clear.
going over 15 degrees on the incline and decline will make it tough to "lock on" and will buck you wildly. If your incline and decline are 20' long then make the flat about 5' above the lip of the incline (that'll be about 15 degrees, but i havent done trig in a while so dont hold me to that!).