Not sure, depends how you want to define biggest.  I have a Churchill Computurn Lathe, its about 3000 KG and have attached the capacity below.
Hood

Chads mill is another "BIGGUN"

http://www.machsupport.com/forum/index.php/topic,627.0.html

RC

My Yamazen mill weighs 4,500 pounds and my Hardinge CHNC weighs 5,000 pounds (but half of that is the stand.)

Vince

Ive done a lathe that will turn 40" dia railroad wheels, and about 10 feet between centers. I also claim its the oldest lathe to be converted- built about 1892

see http://www.machsupport.com/forum/index.php/topic,5654.0.html

Here is another "BRUTE".
http://www.machsupport.com/forum/index.php/topic,5536.0.html
RC

Ron,
 whats the rigidity like, tool seems a bit precarious perched away up there but suppose its taking fairly light cuts on those wheels?

Hood

The big lathe only has a 5hp motor on the spindle, so that really limits our cuts. The tool is actually on 2 compound slides we stacked up. It only covers about 6" is either axis, but for railroad wheels thats all we need. A wheel has a rather complicated profile of tapers and filets, so the cnc really helps, doing them manually was a real pain.

ok,

   Here is the OMAG Granite mill I made the control for, 2KW AC Servos, 20hp spindle, when you click the link
loot at "Reply #1" third picture down. There is a Full Grown 6' man standing on the Mill Table.......



Mabey not the bigges, but envelope on it is 20' in the X, 12' in the Y, 6' in the Z

Has Air, vacuum, and Hydrolic systems, plus the regular Electronic stuff........

Line:   http://www.machsupport.com/forum/index.php/topic,5536.0.html

scott