Robert
There's two kinds of halibut, California and Pacific. Pacifics get really huge, a trophy might be over 200lbs or something. California halibut, which is what we're catching around here, get up to around 60lbs. A solid one would be over 20lbs, a real trophy might be over 40lbs.
Mostly depends on where you are fishing though. The biggest halibut I've ever landed in my tube was 33" but I've also been on the backside of Santa Rosa island and seen a half dozen fish over 20lbs caught in one day up to 44lbs. So it's all relative. My personal best was on another Santa Rosa trip and weighed 31lbs.
A halibut can vary in weight a lot regardless of the length. I caught a 33 incher in the harbor once off Marina 1 that weighed 13lbs. I also caught one 34 incher on the twilight boat at the one mile one evening that probably weighed around 17 or 18lbs. When the halbut start feeding, especially on squid in deeper water they tend to get a lot thicker and weigh more than their inshore, smelt and grunion eating cousins.
I've always hoped to get one over 20lbs in my tube, I think that would be so killer. A guy up the street from me, John Williams, got one in his tube once that was 41 inches long. That's a monster out of a tube.

-Rob