This is a small harvester I tinkered with, it's only 3x3 wide and 4 tall. You do not have to add the water, but I do.

It's pretty awesome because unlike a lot of the other compact harvesters, this gives you room on the side for the melon/pumpkin to fall and collect. - This in turn allows for any size of farm to be made with minimum problems in copying the design.

It is also pretty cheap to make, the most expensive things would be two pistons, a slime ball, and a glowstone block if you don't have pumpkins already.