Mine was a 10mm bolt head on the drain plug.
Nice little plug with O-ring--one of the better factory ones I've come across, sine it's hard to over-tighten something like that with only a 10mm head.
I installed a Fumoto valve at the first oil change--love these things.
All the best,
Shane
Here's a direct link to the one I got for the 1.8 auto...
<02> ('04 - on) Aveo w/Daewoo engine, ('11 - on) Cruze, Sonic, Volt w/1.4L,1.8L,
I got the one with the nipple, simply because a drain tube could be affixed if one wanted to do so.
All the best,
Shane