Transmission Serrated Wrench Size?

I need to change out a transmission and was wondering if someone could tell me what size serrated (torx) I need to remove the drive shafts from the transmission. The parts stores around here can't tell me what size I need and you have to buy them one at a time. I wouldn't mind, but they go for around $12.00 each and they said once they're opened I can't return them. I would hate to get stuck with a something I'll only use once. Thanks in advance.

I changed out the transmissions and found out that it is a 8mm Serrated (Torx) Wrench.