Here's an Idea

Submarine Sandwiches.

Why is that you can only get Submarine sandwiches in x-small and x-large...

How many times have you left 2 inches on the table that you just couldn't finish...exactly.

Sometimes 10" is all a person wants, or 8" and not always 15".

The 4" and the 6" is just too small for some or is just a little too little...in other words maybe they felt like 7" today...

canned rebuttal: So what am I supposed to do, carry a tape measure all the time :-P

No, you build it into your cutting board.

Depending on the size and amount of your sandwiches, you'll need to divide the difference for the amount of inches changed to charge ;-)

If you want to have a little fun, go mathematical with it and get really funky in your sizes, by the inch and everything in-between :-)

Why serve only 6 and 12 inch when you could have a size that fits all :-)