You could find the different between the your shadow and your height and use the statistics you found and apply them to the shadow of the building. Standing closer to the building would get best results because of how the sun shines on it making the shadow as long as it is.
Since the truck is 8 feet long then the longest log able to fit in the truck should be 8 feet as well. However talking reality, we might want to shorten it to make it fit better. Hope this helps!
