Size - all of the TC drain plugs I've seen have been 5mm allens.
How to get it in position. Wrench the engine over by hand using a 27mm socket on the crank bolt. Be sure to turn the engine clockwise. I've found the I can lay on my back, feel for the plug through the window with my left hand while wrenching the engine over with my right. Gets the plug centered every time. Trying to get the plug in view using the starter is like trying to hit big on a slot machine - it just don't happen too often.
How much fluid is in the TC, probably about half of the total fluid amount. 3-4 quarts on both of my cars.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.