believe it's a 3/8, but possibly a 1/2. I use a ratchet with 3" extention. The extention just goes into the square hole..lefty loosey, righty tighty.
I suggest getting a hand pump, and pump out all the fluid that you can, and replace it with new fluid. Might as well...it probably hasn't ever been changed. My truck only had like 84k on it and was UGLY!! was milky creaming green lookin' crap! It was sweeeet. lol.