Using Google Earth, you can easily measure hills. Just select "Tools -> Ruler" from the main menu, and choose "Path" from the dialog that pops up.

Now, you can zoom right up to a hill and measure it's width by clicking twice - once on either side of the hill.

Length is also easy - just click a number of times to draw a line down the hill. Unfortunately, at this time Google Earth does not support giving you the measurement of an existing path. You have to use the ruler to draw over your existing one.

You could also go to the hill with a car and measuring tape and measure it manually by driving the distance and checking the odometer.