This goes through it quickly but the principals are quite simple:
http://www.youtube.com/watch?v=RcOtFcIGuXw
You want magnets that fit. There are too many different sizes to list and at least a couple dozen that will work for what you want. Take some measurements and see what size you need. My guess would be a model that size can fairly easily fit 1/4" in the arm sockets but there are many different thicknesses and strengths as well as shapes that will work. I tend to keep several different sized magnets on hand for my needs.
Generally I find 1/8" to be the most common and have both 1/16" and 1/32" thicknesses on hand. For larger models or vehicles I keep 1/4" around as well. I also have a few 3/8" that don't see much use.