It was a learning curve for me too....if you tighten the top first then you will have trouble getting the alignment right....you basically have to tighten both almost all the way down and then play with it until it is positioned right. I wish I could offer more advice but it took me a couple of tries to get mine right.
They are about 3 inches at the tallest point and 5 inches at the widest. Mine came with an extra adapter bolt that was reverse threaded for the left side so when the wind hits it it tightens it up more. I also used 271 Red Loctite on both the mirror studs and the adapters.