The longest bar I have seen was 12'. It was driven by an electric motor and operated via hydraulic lowering cylinder like a giant chop saw. That was back around 1972 or 1973. The saw was used for slabbing redwood stumps. The table tops that came out of the shop the 2 owners ran were beautiful! I got involved because they wanted to buy a used fire engine so they could wash the stumps off where they were dug up.
We had a picture for years in the shop of two guys running a model 99 Mac with a 14' bar on it, dropping big Redwood. Longest bar I've seen first hand was 96", and the guy that owned that did quartering of big logs that came into the local mill in Central Point, Or. Longest bar I've run was 72" on a Stihl 076, working up big Port Orford Cedar.
Well Bill, it depends on what motor you're looking for long bars for. I have a buddy, a retired log cutter turned machinist/fabricator that has about 600 bars for early 60's to present saws. I know he's got some 7 and 8 footers in there...