For sump sizing you first need to have an idea of what skimmer you want to make sure you have enough room to support its footprint. Another thing I always consider when designing a sump is that I ensure there is enough spare room to hold the water from the display tank that will siphon back in a power outage. Some people rely on check valves for this but they are highly prone to failure and require frequent cleaning or they will bypass. Check valves are handy for doing sump maintenance but I personally like the peace of mind when I'm not at home that my tank won't flood my house. Filter sock holders are also really nice to have in the sump but take up more space. The return section of the sump is the only chamber that will fluctuate due to evaporation. If you are using an automatic top-off system or not will determine how big you want to make this section of the sump. If you make it bigger you can still add the ATO at a later date.
Flow through the sump doesn't have to be great, around 5x the tank volume is ample. I had a Mag9.5 in my 90g that worked great. The return pump shouldn't be your primary source of water movement in the display tank, use powerheads or some type of wavemaker for this.
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