i would stick to around 3-4 in. a true DSB
is around 7 or 8 in., but they also come with quite a bit of baggage, esp for the new aquarist- they can actually become anaerobic in areas, and if those areas get disturbed they can release poisionous hydrogen sulfide gas which will result in fish death. DSBs are also linked to large algae blooms since they can become nutrient sinks, this is just IME
, i do know people with very sucessful DSB
but yea 3-4 should be good. a nice area for denitrifying bacteria to grow.