I'm on the verge of ordering some sheet magnets to magnetize the bases of my army (for ease of storage & transportation). I still have a lot of infantry on long bases and some tanks on 40x20 warmaster bases. I was wondering about the precise dimensions of the "long" infantry bases, at the widest point (where it's touching the table).

I'd say 12mm x 40mm, but I'm not quite sure.

Do anyone know the dimensions from the top of his head ?

I believe it's 40mm x 12mm as you say.

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Currently doing a plastic scenery kickstarter

Thank you all ! finally I have opted for sheets of adhesive magnets, because they are thinner (0.85mm) and as I will end with quite some variety in my bases (notably round bases for my eldar army and my new marines).
I Think these A4 sheets will be easy to work with.

The best source for magnets I know is http://www.first4magnets.com (for Europe, they're based in the UK).

The prices are like 1/5th of the first site I ordered from, and they have an incredibly wide range, including some 0.5mm thick magnets, not so easy to find elsewhere.

I even picked up some "toy" magnet cubes to use as semi-geeky gifts for Christmas . Kept a copper red one for myself of course

I used 2mm diameter, 1mm thick magnet, so far they are perfect for resin titan weapons (They even keep some poseability with FW-like ball joints), but i got some other sizes to experiment other stuffs as well. The 6mm diameter/0.5mm thickness will be the perfect stuff for my 6mm Space hulk

I used Litko magnets to cover the bases of infantry I already have.

I use www.shogunminiatures.com for new bases, because they are cheaper than bases+magnets bought individually.