It's about half way through the field at a daily live $125 buy in tournament. 
The blinds/antes are at 300/600/75.  There are about 30 players left
from about 60 entrants total.  Five spots are being paid, so I would
definitely quantify this as the mid stages of the tournament.  Average
stack at this point is just under 15,000.

Villain in the hand has done nothing remarkable since he joined the table maybe about halfway
through the second level.  He hasn't done anything that appears to be a
move and seems fairly straightforward.  Based on the hands he has shown I
would definitely classify him as loose.  He also seems to not deal too
well with aggression.  FWIW, I think my table image is fairly tight, I'm
above average chips thanks mostly to a hand where I flopped the nut
straight against another player's flopped set.  I've also been fairly
card dead and therefore haven't been too active.

Stacks at the start of the hand, V has 12,500 (~21bb) and I have 20,100 (~33bb).

Pot size preflop is 1650.

V opens UTG to 1800 (3x).  Folds to me in the cutoff and I have AKos.

I
felt this was an appropriate spot for a three-bet, and if there's
debate on that I think it would be worth discussing, but the main thing I
want to talk about is bet sizing for the three bet.  In this hand,
given 21bb effective stacks, what is your preferred size for a three bet
and why?

I'd also open that up to general discussion of bet
sizing on three bets.  We're all familiar with “standard” sizes for
opening pots, but what factors do you take into consideration when
deciding how big to three bet?