1/4" and 3/8" and 10mm are common.
There are a number of special bleeder wrenches available
http://buy1.snapon.com/catalog/tools.asp?tool=all&Group_ID=1686&store=snapon-store
If it fits a good 6 point is all you need, or a Flare Nut (line wrench) works too but the open part is unnecessary for bleeder screws, and a box end is stronger.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-