Squier uses 4mm.
Use a ball end hex key. You don't have to be straight-on with it then as it'll bite even if it's not all the way in - not having a good bite is probably the #2 cause of stripped truss rod nuts. #1 is using a key that is close, but not quite, the right size.
I vote flashlight as well. That's 3 of us so now you're going to have to do it ;P
Remember: righty-tighty lefty-loosey. Tightening adds backbow to counter the string tension, loosening removes backbow and lets the strings pull the neck forward. If the neck weren't under so much stress I'd say to always tighten (like when tuning), but it is, so letting it out isn't so bad. Either way, make SMALL adjustments over time. Don't turn it more than about a quarter turn at once before giving it a day or so to settle.
Squier VMTB & SBCV PBass.. my wife has a Bronco. CV club #169. TBass club #78.
Last edited by gurensan : 08-15-2013 at 08:25 PM.