There is a particular Garmin GPS unit that does area calculations very nicely although you really have to hunt through the manual to find out how to do it. It is not one of the main menu items. This is actually one of their lower cost models but not all models will do this area calculation.

I had a salesman at the place I bought one (Cabela's) show me how to do it - he says he sells a lot of this particular model for exactly that purpose. You find this area calculation, push start, walk or ride around the field back to the same point, push stop and it gives you the exact area you've enclosed. If you let it sit on the start point (I used a corner fencepost) for a few minutes the accuracy of the whole calculation gets better.

Jim at Dawn