I know that size matters, but I prefer several little magnets, to one big one. I have about a dozen ceramic magnets, (about 2 1/2" X 1" X 3/8") and about 20 rare earth magnets, (about 1" X 1" X 1/8"). Even separating one of the ceramic magnets from the stack, is something done with a great deal of caution. And getting one of the rare earth magnets to separate can easily result in injury. My reason for choosing to go with multiple magnets, over single larger magnets, is that its more convenient to be able to use them stacked, or separate them and lay them side by side on a piece of steel, with opposing poles facing each direction, N-S-N-S-N-S etc... When laid out as mentioned, they work great under a sluice box, but they do tend to leave a pattern in the black sand collected.
The reason for using steel backing the magnets in the above application, is that it keeps the magnets nicely in line and it increases the gauss (magnetic field strength), similar to the magnets used in stereo speakers, (another source for magnets). From what I've read on the subject, steel backing of magnets approximately doubles the gauss of the magnet, (but don't quote me on that).
As mentioned, it is a good idea to use something between your magnets and the items they may come in contact with. Making some single, or double plastic covers for your magnets from lexan/plexiglass, or other similar materials can save a lot of time consuming cleaning and possible damage. For example, someone who wants to use a barrel shaped magnet for removing black sand from concentrates, could use a pill bottle, where the magnet can fit inside the smaller pill bottle. Gluing, or otherwise attaching a piece of wire to the magnet as it sits in the bottom of the bottle and having the wire extend out through a hole in the top of the bottle, would allow you to pick up iron then release it by lifting the wire/magnet, (causing an increase in distance between the iron and magnet), enough to drop the iron elsewhere. I used to see a device like this advertised for sale, but that was many years ago.
F.