found this calculator very useful, you can work it all out yourself;

http://www.kjmagnetics.com/calculato...calcType=block

my magnet was N45grade 50X50X20mm- 5mm ground clearance, then mounted on 2mm ti base, giving a distance of 7mm from the steel plate (arena floor) its being attracted to.

I wouldnt suggest you bother getting magnets if you have a standard drill drive, your wasting your time and asking for trouble.
The location of magnet relative to the wheels is very important... for example if you had a wedge shape robot with wheels at the back, magnet by the flipper tip... it would take the traction away from your wheels and you'd probably just sit wheel spinning.

Ideally you want a magnet next to each wheel... or if this isnt possibly, bang on in the centre of the wheel base, making sure your base is rigid enough not to flex, but obviously none magnetic so the attraction isnt effected. Have fun!