If you know the area of the map in total or can cut out a rect of known area around the complicated bit then you can start with that. Then add a layer, trace around the part and fill it black and white and dump the original.
With the black and white version you can use a mega blur on it. The one I like in PSP is under Effects / Geometric Effects / Pixellate and do it as much as it will allow. Once the whole screen is one uniform gray color then get the color value and divide it by 255 and multiply by original area to give approx area of complicated shape. That should work to about 1% - 2% of actual accuracy due to errors in rounding.