Well an area with no trees to the east, south, or north is the ideal location. What you would want to do it determine the heights of the trees then use geometry to determine the length of the shadows. If your in the northern hemisphere the length will be greatest on December 21.
However do bother trying to do analysis on how much sunlight your going to get. Just do a Google search for something like hours of usable solar hours by latitude. The number of usable hours is going to vary by your latitude. There are sites out there that will tell you the number of hours based on your location. Where I live in Minnesota (USA) we average 4.2 hours a day. With higher numbers in the summer and down to 1.9 in the winter. It does not matter where in town you place it the hours will be the same. Unless if you have LIDAR data available and 3D analyst you can work out some calculations on shadow length, but you will still need to find the angle of inclination.