How deep should guitar nut slots be?
The slots should be only deep enough so 1/3 of the string is above the top of the nut and 2/3 in the nut.
Can nut slots be too wide?
The strings may still bear at the front edge, be supported along the entire slot, be at the right height and spaced evenly. If your nut has deep grooves that are too wide, and the whole thing looks messy, don't fret.