This is actually something that has been quantified recently. And, it's been done for the whole globe! Really creative folks have used satellite-based LIDAR and RADAR to measure the forest canopy height and have provided maps that you can iteratively interrogate to see what the height of trees are anywhere:
Global Forest Heights
For those of you who would like to compare the height of your forests to those favorites of Mitt Romney:
Credit: cropped from NASA Earth Observatory/Image by Jesse Allen and Robert Simmon/Based on data from Michael Lefsky.