For a 90 gal
tank I would have a sump no smaller than 30 gal
. If you where to have a seperate fuge I would shoot for 20 gal
There is no real ratio or formula for figuring this out but I would stay a good figure to work with is the sump should be at least 20% of the tank's capacity and the same for the fuge if the fuge is seperate of the sump. The fuge should have nominal flow of lests say 5-8X the tank capacity per hour and the sump should be between 10-15X the tanks capacity. Those flow rates would permit you to have a tank with no or minimal powerheads inside the tank. Having lower flow in the sump and fuge will increase the need for powerheads in the main tank.