3.8 cm stents were approved for use in Feb 09. Trying to navigate them to the coronary arteries without knocking off plaque on the way must be a nightmare.
I looked up the length of my stents, I've got a bunch, and the longest I have is twentyfive mm. Like Ed34, it bridges two stents, one 20mm and on 12 mm. These are in my RCA.
There probably isn't a limit in reality, it's more likely to do with how much the Cardiologist is comfortable in doing. My Whole LAD was stented last year, it took five long stents. These are all end to end.