It is a volume equation. You will want at least a 1:1 ratio FT:GB. You can stock fish at a higher level and go 1:3 giving you more filtration.
I'm used to seeing the ration as
Grow bed to fish tank
Anyway. I would probably fill an IBC to around 200-225 gallons so then if it was a simple system and only stocked lightly I would then use a second IBC cut in half as the grow beds so it would be roughly a 1:1 system or equal volume of fish tank and grow bed.
If I had space for a sump tank or budget to use an indexing valve, I would set up with twice as much grow bed as fish tank or a 2:1 grow bed to fish tank ratio (this ration makes sure that you have enough filtration for the max number of fish you can likely get away with in a given amount of water without oxygen injection.
If you go with the 1:1 grow bed to fish tank or less grow bed you have to be careful not to put too many fish in your tank because you don't have enough filtration for a heavily stocked tank.
for 200 gallon tank you would need 26 cu. ft of grow-bed
Or if it's easier, for a 200 gallon tank, start with 200 gallons of grow bed. If you have some means of mitigating water level fluctuations you can go up to 400 gallons of grow bed or even more.