It was a small, fat one, I want to say a #23. But I did that job several months ago, so my memory of exactly what size it is isn't that great-- could have just as easily been a #32.
My injector has a lip on it to hold the O-ring on, IIRC. The injector O-rings are the only ones that are semi-critical, since the AIS/TPS ones are more for dust and vacuum sealing. The rubber that the generic O-rings sold at hardware stores are made of are pretty elastic, and will stretch easily, so you can get a slightly undersized O-ring, preferably a thinner one, for those if you can't get an exact fit.