I'd probably be tempted to size the chain long enough so that there's just enough derailleur cage hang/clearance in the smallest sprocket (so, bordering on too long). You should have way more than enough chain for the large sprocket (probably more than you'd choose to have), but I don't think more chain than necessary has many drawbacks, at least assuming it's not so long that it drags on the rear derailleur cage. But we've already checked that, so that should be good. Your XT rear derailleur should provide plenty of chain tension. In theory, a longer chain will have more wear life and I think the drivetrain seems to run smoother with lower tension. You can always shorten the chain later (or install a chain device) if you have problems with chain drops.