17mm
you can get a purpose built socket tool like this;
http://www2.cip1.com/ProductDetails.asp?ProductCode=ACC%2DC10%2D7165

or
find a 17mm allen wrench, cut about 1 inch off with a bandsaw/cutting wheel then use the stub along with standard 17mm wrench/socket to get the plugs out.

or
you can weld a 17mm nut to a short 17mm bolt if you have a welder and make your own tool.

Always good idea to loosen fill plug first, make sure it is not seized before you drain all the fluid and find out no easy way to put new fluid in.

---------- Original Message ----------------------------------
From:         Michael Sullivan <sandwichhead@GMAIL.COM>
Reply-To:     Michael Sullivan <sandwichhead@GMAIL.COM>
Date:          Sun, 14 Mar 2010 11:36:19 -0500

>Can anyone tell me the size of the drain plug and refill plug?  Auto.
>Thanks.
>Michael in San Antonio
>91GL AT 'Gringo'
>73 Beetle
>