For most areas around here, I can get better (newer and higher resolution) photos through the county auditor's websites. Also free. These give parcel information, and show where the property lines are, so if available, are more useful to me than Google Earth. For areas with no photos on the auditor's site, Google Earth is a great option.
I can see where this would help you see how densly wooded an area is, but it won't tell you much for diameters. Is it thick with 8" diameter trees or 28" diameter trees? If you have a nearby area that you are familiar with, you can certainly get some relative perspective...