Sump pump size...

[MKLKT]

Generally you want the turnover in your tank to be 10:1 or greater from return rate:tank volume. Remember you have to take into account the head height for the returns of the water being pumped back up from the sump to. You'd probably want to play with baffle ideas to put a fuge in that (low flow is better for refugiums.)

For example I have a 65g display with a 20g sump and I have a pump that outputs ~950GPH at 4' which is the vertical distance the water is traveling.