I believe he had some stats done up in the 1st edition Rogues' Gallery, but they really wouldn't do him justice in 3.5. At best, they'd give some idea of his development on the path to greatness, but that's about all.
In the City of Greyhawk box (Folks,Feuds and Factions book), Bigby is statted on page 22. He is level 18, while Mordy is level 20 (and statted a few pages earlier). The whole Circle of Eight (c. 580 CY) is statted therein. Bigby is said there to rarely adventure now, so he won´t gain too many levels anymore - just barely epic, I´d say, if even that. It would be perfectly ok to leave him at level 18 to 20 IMO.
In the City of Greyhawk box (Folks,Feuds and Factions book), Bigby is statted on page 22. He is level 18, while Mordy is level 20 (and statted a few pages earlier). The whole Circle of Eight (c. 580 CY) is statted therein. Bigby is said there to rarely adventure now, so he won´t gain too many levels anymore - just barely epic, I´d say, if even that. It would be perfectly ok to leave him at level 18 to 20 IMO.
In addition, you might want to email Crieghton of the LG Circle. He probably has Bigby statted out for use in Living Greyhawk modules, or knows which region to contact to get those stats.