what size reservoir?

[TheMcs]

Here's a quick rundown;
Sequence Hammerhead (5600 gph) feeding a 185 display & 200 frag tank.
3 - 1.5" drains on the 185;
1 to the skimmer & fuge
1 to the 200
1 to the reservoir for a closed loop effect

the fuge & 200 will also drain to the reservoir
the reservoir will feed the pump & serve as my topoff location.

Now, what size reservoir do I need? I'm not sure what size drains for the 200, I'm thinking 2 - 1.5" drains.

Any ideas?