2mm deep is perfect for underneath bases, the 4mm width fits nicely into the various gaps on all GW square slotta bases. Magnets are strong enough to hold metal figs in place but I wouldn't want to turn the tray upside down as I can with some larger magnets, but this makes model removal easier which is a bonus.

Perfect for my needs.

Powerful magnets and at a useful size as 4mm drill bits are quite easy to come across. I find it best to drill a hole for the magnet and then superglue it in place- makes for a more secure and neater join. I have used my magnets variously for: the weapons on my Imperial Guard Valkyries with a magnet in each weapon and a simple poster pin/ thumb tack glued into the weapon mounting, to attach models to a scenic base so I can take them off for separate use and to magnetise arms for some of my models so I can swap between different weapon combinations. These are strong enough that you can get by with most jobs using a single magnet on one side and a piece of magnetic metal on the other, like the aforementioned pin. All in all a very handy tool for any keen converter and a good means of getting more out of your models with extra equipment options that you'd otherwise have had to glue permanently.