You should use 6x2 N52 magnets in all big cubes. JK. What 7x7, 8x8, and 9x9 do you plan to magnetize? If it is like a small mf9, then you should go for 3x2 or 3x1, or maybe even 2x1. N35, or N50 should work just fine, and both of those can give you a quite strong feel based on where you put the magnet. If you are working with a shengshou 7x7 on the other hand, then you are better off going for bigger magnets, because those have giant layers. Magnets can be bought on ebay for way cheap, but they take a while to ship. Hopefully that helps.