I've seen a few different sizes - check the DIY Dock thread and you'll see some pictures. I initially tested it with a magnet off my refrigerator, so it doesn't need to be that strong.
There are 2 locations, one for the Home and one for the Nav. The magnet in my old case for my old phone was powerful enough to trigger it. You might just go by Radio Shack and try different magnets and move it around on the back of your phone until the screen you want comes up then mark that location on your phone.
I would be careful and not use a very strong magnet. A rear earth magnet is very strong and could do damage to the magnetic switch (if there is one) in the phone. I would dont think I would want overkill on this particular situtation. I was holding my handheld scanner and my phone in one hand the other day and the homm dock popped up for a second. About the time I said "what the heck?" I realized what happened. Hehehe