The record (I think) is held by Josh Simpson - who under a grant by Corning - created a 100 lb+ planet a few years ago. There is a DVD of it - well worth watching. It took him many tries over the span of a year, changes in technique and a lot of innovation. On the more sane front - Josh and others regularly turn out 5-8" diameter weights... up to 10" dia (the latter being more corporate display pieces IMO).
The problem (I think) with very large weights is both technical and artistic....more of the latter I think. The technical is just mechanics, weight on the punty and a longer anneal. As a weight gets larger - controlling the design very well becomes more difficult.
I own large Simpsons in the 3.5" range, Matthews at 3.25", and Harry Besset and Lundberg pieces > 3" dia. - so my sense is that is around the larger sizes targeted by many artists.
Here are some really large Simpson pieces - the largest being around 10" dia.:
Alan