i would make the long ones only and placemats. price the long ones a little higher than you said and price the mats as a set of four for a little more. offer additional sets of two for slightly more than half price of four.
edit: you could take orders for shorter ones if you like for more than half the cost of half the length. have a nice sign for that, but don't show them.
edit edit: i think the mats will surprise you.