I usually do it by using the windows and doors as a guide. If you can guess the size of the doors and windows, then you can get pretty close to the dimensions of the building. For example, on residences, you can figure on personnel doors to be about 6'-8" tall and on commercial buildings, they are generally 7 ft tall. Of course there are exceptions to every generalization. If the picture has a person standing next to it or possibly something else that you can guess a dimension from. If it's a brick building, you can count the number of courses of brick. If it's standard size brick, then 3 courses of brick would be 8 inches.
Mike