How to choose a reed switch and magnet?
One of my school projects required magnetic sensing, and using a solid state hall switch beat out a mechanical reed switch in terms of price, size and reliability. I recklessly ordered high-sensitivity switches and used a really strong neodymium magnet which did work (perhaps a little too well, turns out the pull up resistors were magnetic too!) but obviously this is a risky approach. 

If you have an unknown magnet, you can use a gaussmeter to measure the magnetic flux density at the approximate distance you want to trip and buy a sensor that has an operating point slightly less than that in the worst case. If you're buying magnets, then you want to use a magnet calculator like this one to calculate how strong your magnetic field will be, given the dimensions you've specified for the magnet and the point to measure at, and then buy a sensor in line with that. Magnetic flux densities and operating points are specified in either gauss or millitesla. 

NB, the Melexis US5781, a variant of the one I used, has an operating point of 15mT (150 gauss). That's low as far as magnets go. It pulls down as long as the magnet is in proximity.