18mm x 2.94 mm (thick) Its a strong magnet. I'm not sure you will find a stronger one. Depending on battery length you may be able to put two magnets in there.
When I got the light to work and stuck it to the side of an electrical cabinet it stuck quite well. I was a bit concerned when it slid down my fridge. I think this is more of a reflection of the quality of the metal in the fridge than the magnet.