I once saw a Jingal replica and the man who made it was around 5ft 7ins tall. It was....1979 (!!) and I thought it was a joke at first. "They couldn't fire them standing or kneeling without support" he told us "They are called 'wall guns' because they were rested on walls" The gun would have been around the 7ft length mark. Compared to the Japanse and average height of time then, yes, those jingals must be 7 feet at least.
Incredible the powder weapons they came up with. Some are mentioned on this Wiki entry. Lots of scope for gamers!