You want to have at least 10% of display volume in the fuge. More is defiantly better also.
Second thing is you will need actual 10x display gallons per hour water going through the chaeto. This means that if your return pump is rated at 10x's it will not be enough because head loss, reduced flow, due to friction and height.
Do you have your skimmer yet? I would wait until you have your skimmer that way you will ensure that it fits and by making the fuge section for the skimmer it's possible to maximize the fuge space.
If you haven't chosen a skimmer yet there are some options to consider. Space saver skimmers (SSS), pump inside skimmer body, will have the smallest foot print. However because the pump is in the body they have the least air/water mixing volume. The pump takes up some of that volume. Still you can have a smaller skimmer section and larger fuge area.
Skimmers work by making bubbles. Organics stick to the outside of the bubbles. Organics rise to the top via a ride on the bubble and eventually reach the top and overflow into the cup. It's important to have a good volume in the body for contact time for the organics to stick to the bubbles. The more volume increase contact time and performance. This is a simple explanation, hope it helps.
Personally and many know this but external or recirculating skimmers perform the best and obviously can be outside the sump or inside. External skimmers have better water level control, more air/water mixing volume equals more contact time and performance. If possible put the skimmer outside the sump and make a bigger fuge.
A good fuge is importAnt to Triton.
Size
Flow
Lighting
Get these right and chaeto becomes a real export method. This is due to the use of new powerful LED grow lights that maximize spectrum for best growth.
This is how much chaeto I harvest from my 30 gallon fuge every two weeks.