I have a Garmin GPS MAP 76.  It measures area (acres).  To measure area, you go to the Main menu, then go to Tracks.  Then just trace the acrea to be measured, and it will calculate the acres.  You can store up to 10 tracks in its computer, so you can store up to (10) area/acre measurements; after that, you need to delete them in order to store new ones. 

I suggest going to the Garmin website and checking out their GPS models. I think many Garmins will measure area, but I don't know off-hand if all do.  Look at each product; under the spec tab, check out features, and it will tell you whether it measures acres.

Some Magellan GPS units measure area, but not all do.  In fact, I think most of the lower end Magellan's do not measure area.  If I had to pick out one brand of GPS to measure area, I would look at the Garmin first.

If you go to the Garmin website, click on several GPS units to compare.  When the comparison table comes up, scroll down thru the list of features.  Part-way down the list, it will tell you whether it does area calculation - that is what you are looking for.

If you decide to purchase a GPS unit, look for one with a USB interface, not a serial interface.  You will make life easier for yourself in the long run.  (again, you will see this feature in the comparison table.)