You'll need to use a measuring device. The cheapest is a fiberglass surveyors pole, which will go up to 40' in the air. You will need 2 people. One raises the pole to the height of the bottom of the proposed sign, and then holds the pole and the other drives up and down the road and photographs it. You can then draw on the sign, to scale, so see what it would look like. The other option is a skyhook crane. Same methodology.