Typically you should count on around 500M for the application binaries etc and another 5-10G for the database as a minimum. Beyond that it's really driven by the number of records to be loaded into the instrument and ledger stat tables. Normally (rule of thumb) is 2K per instrument record so if you are loading 1 million customer rows then ballpark 2Gb required. Of course if you are keeping history then you'll need to factor the same x load frequency x how ever many years of history you want.
Most OFSA users are around 100-200Gb in thier databases though I've seen some MUCH bigger (> 10Tb)