If you want 5 gallons of finished beer, you have to have some extra to account for trub and racking loss. You generally start with 6.5 gallons of wort, boil it down to 5.5, transfer 5.25 to the fermenter (leave hops and break in the kettle), and rack 5 into your keg or bottling bucket (leave yeast and trub in fermenter).
For an imperial stout you may want to employ a secondary... scratch that, let's call it what it is, a brite beer tank. RIS is a style that needs lots of time to mellow. Hint: kegs make FANTASTIC BBTs.
Keg: Simple AIPA (2-row, Chinook, Cascade, WLP090)