That's a great shot of Mr. Nasty that Keith put up in his post!
The shot Keith had is probably about as large as they get, but I'm sure some individual snakes get to be somewhat bigger.
78 or so inches and 7 lbs would be my guess as the max. you're going to get on a Florida king unless it's an obese snake.
Everyone seems to know someone who knows someone who's seen larger kings, but the proof seems to be lacking in most cases.
The largest Florida king I've seen was about the size of the one Keith posted - it was (two years ago) at the Disney Nature Preserve outside of Orlando, run by TNC; they also have the largest Eastern/Eastern-Florida intergrade I've ever personally seen - that snake was a freakin' beast with the largest head and thickness I've ever seen on any type of kingsnake.
I'll try to find a picture of my largest Florida king, "Loco," a "brooksi" that's 64" long and about 4.5 lbs; he's about 6 years old and this will give you another shot of a large floridana. Others on here will surely put some pics up of their large Florida kings too.
-John