I just did this project and used the largest disk magnets that HD sold. I think about 2.5" in diameter. I used 5 across the top and 2 on the bottom corners, and I still wish they were more powerful, but they are working. Hopefully you get the info you are looking for but If there is a choice I would look for the most powerful you could buy. Mine are probably not as strong as the ones Magnashade uses.
My magnets hide behind the black ring of the windshield so very little of the magnet is visible from the outside when the shade is removed. I held mine on with silicone sealant so I could remove them like someone else suggested.
I have someone else in the park with a Magnashade and we both have about the same luck when a storm comes up. These do get blown off once in a while but only with large gusts probably ~50mph. I still love being able to just push it into position with an awning rod or broom handle.