A question came up concerning the size of the fuel tanks on a deckplan I posted recently, so I decided to do a quick reality check to be sure I hadn't completely mismeasured them (I hadn't, although my eyeball estimate was about 20% high). So I needed to find a way to discover the area of an irregular shape. A jaunt over to the Adobe forums came up with this cool trick:
Select the object you want to find the area of. If it's part of a larger object, just trace over it with the Pen tool so you have a single shape that is the size and shape you need.
Ctrl-Alt-Shift-F12 (Cmd-Option-Shift-F12 on the Mac) will call up a debugging window. I'm sure this window has lots of awesome info in it, but I haven't really explored it much yet. For now, scroll down to the section called "object tree." All of the objects in your drawing are listed here, but instead of having convenient names, they're all called "path" and given a hexadecimal identifier. However, the currently selected object will be in bold type. If you don't see it, copy and paste the object so it will show up at the front of the list. You can delete the copy later, of course.
Now click on the bolded word "path." Scroll back up to the "objects" section, which should be directly above the object tree. That section should now have a lot of data about your object, including its area measured in points squared.
Obviously, in order to convert that to a unit that makes sense for your map, you'll need to know the relationship between points and your units. The simplest way to do that is to make a reference square that is one unit to each side. You can then find out the area of that square in the same fashion, and that will tell you how many points are in your unit. After that, it's simply a matter of dividing the area of your object by that constant, and you'll have the area of your object.