There are probably volume calculators on the internet. Your gravel bed water volume will be approx 40%. For example, a 100 gallon gravel bed holds approximately 40 gallons of water. If you have or intend to have one such gravel bed, then a 50 gallon sump would be sufficient.
George is correct but I'd go with a little larger sump, maybe 75 gallons so you don't have to keep close tabs on water loss.