I would get a really big one; you can't have too much media capacity. I'd also use the new schuran media and a dwyer flowmeter to regulate the CO2 flow. You can then get any cheap CO2 regulator that drops the supply pressure to 25psi.

The size of the reactor determines your contact time with the media, which directly affects the effluent dkh/ph ratio. That ratio is the primary determinant of the reactor's efficiency. There's absolutely no problem running a 3 foot tall 6" tube reactor on a 25 gallon tank if you want to; you just dial down the flow and the CO2. What's very important with calcium reactors is controlling the CO2 and effluent flow rates.