I say if you're going to get one, get a big one. Take good care of it, especially if you wear shorts a lot you will need to be very good about using sunscreen on it. The artist will probably include a touch up down the road to fill in any missed spots or places the ink didn't take as well, but make sure you discuss it now.
One of the nice things about running half marathons is that I spend plenty of time in shorts with people behind me with nothing to do but chatter. Get plenty of compliments.
Here's the gecko on my calf:
And the piece on my back (it's about 4 inches in diameter):
