Mike,
All you have to do is measure the width of your boat at the area you want shaded, and then decide the height you want. I just installed one on my boat (pictures in my library). I measured the depth from floor to top of rail, subtracted that from my height and got the smallest size so I don't have to stoop when I am under it. I also measured to make sure that when it is not in use, it lays behind the rear casting seat. The fishing god's smiled on me and everything lay exactly how I wanted, but Cabela's and Overton's also sell sliding rails that will allow you to, if necessary, slide it out of the way.
If you can read a tape measure, work a drill and turn a screw, you can do the install yourself. It took me about an hour to do.
Feel free to email me if you have any questions.
Rich D