I magnetised some power armour infantry and a rhino using magnets with a diameter of 2mm and a thickness of 1mm. For dreads, you problably want to use some bigger ones since the arms will be heavier. Same with terminators.
For example:
Power armour: 2mm/dia, 1mm thick
Terminator: 2,5mm/dia, 1,5 thick
Dread & Mech: 3mm/dia, 1,5-2 thick
Be sure you buy neodymium (aka 'rare earth') magnets, as they are the ones you need.
Go here for magnetising dreads. (Also look at the other magnetising articles)
http://fromthewarp.blogspot.be/2010/12/magnetising-space-marine-dreadnought.html
This tutorial is quite good for infantry.
http://www.dakkadakka.com/dakkaforum/posts/list/427173.page#3994385
Hope this helps a bit,
MC