[V8] What size hex on 5-speed tranny filler?

[toml99 at todomundo.com]

Hey, while I was under there today, wanted to check the tranny  
fluid.  Car was refilled w/ Redline just before me getting it, and it  
doesn't leak, but just wanted to check it.  Assuming I'm going to  
take off the upper hex plug(drivers side) and feel for fluid?  I  
didn't possess the large enough hex, and was wondering if anyone  
know's the size?....Tom