As small as machinably possible. You want to have it as close as possible WITH CONSIDERATION for changes in size because of temperature change, and wobble (if its not 100% exactly right) you get exploded bits. Closer magnets and coils = better magnetic flux continuity = better efficiency. If you can machine it to a tolerance of less than a millimeter so much the better, but you'd better be darn sure about it holding up to strain and not contacting. Tips to get it closer: If its a disk-brake style on a flat plate of magnets/coils, get your plates as flat and smooth as possible so they can be close. If it is a drum-brake style 'cage' of magnets around a stator, then use specialty rounded magnets to make a true inside diameter circle. If thats not an option (too expensive) use more, smaller magnets in clusters. This has the same/more effective magnet area, but more sides on the proverbial polygon to make a rounder surface.