The snarky reply is look around you and divide by 220! However, as you say you're not surrounded by old growth trees. In our neck of the woods (S.E. Michigan) typical mature trees vary from 50ft to 100ft high depending of species. Roughly 2 1/2 to 5 inches in z scale. Now, if you're modeling California Redwoods on the other hand....

My opinion is to use trees that appear realistic in height on your layout - not out of place. Even 5 inches is a too big for z scale in my view - think "selective compression" here. More often than not this means trees much shorter than scale height. California Redwoods for example. Who wants 12 to 18 inch tall trees on their z scale layout? They would be overwhelming!

Now if you had a warehouse for your layout...