A CR123(a) is also a 16340 battery. Means 16mm diameter, 34.0mm long.
If you want really small, what about the AAA sized 104400? That's 10mm diameter, 44.0mm long. They'll fit in a much smaller area than a pair of 16340 although the space will need to be taller.
I don't know the equivalent to the CR2 size. If I did you'd have the answer to your question.
Ack! I misread your post. You want 3.0V versions for an unregulated 6V
mod not a 5V regulated mod. My suggestion for 10440 won't apply but I'll leave it in case you think about going the 5V regulated way instead.