How tall can you go. Remember, Mangroves are trees. If you only want the Mangroves to function like an "algae scrubber" to reduce nitrate, there are easier/cheaper ways to do it.
There are hundreds of sump designs esp. DIY
ones. A seaarch of the forum and/or net might help.
Include these criterea:
Provide low flow across the refugium portion. Include baffles before the return pump to eliminate microbubbles. Make the sump portion large enough to hold a heater and skimmer. Include a float switch for an auto top off scheme (this is the main advantage of a sump IMO
). Use an external pump if possible to reduce heating the water from the pump motor.