Actually, it should be kinda small. Your terrain has a surface area of 16,900,000,000 square feet (606.2 square miles) and a mid size tree, when looked at from above as a leafy circle of radius, oh, say, ten feet, is going to make a shadow about 314 square feet. The tree, from above, is going to occupy 0.000001859% of the surface area of your terrain, which is tiny indeed.

Try starting with a medium size scene instead of a large one, and add a terrain to that. The default scale is much closer to the size of a tree (that is, a tree fills the camera view much more completely) and you may get closer to what you're wanting.