I did some close photo comparisons a while back (mostly using behind the scenes pics of Bib & Jabba and Bib and Luke) and Jabba seemed to be about 5'-6" (so just under Luke's height,) which would make him 11" tall in 1/6, maybe a bit over.
The old Sideshow Jabba is a whisker under 10 3/4" and the new Sideshow Jabba seems to be at least 11 1/2" (the height listed on the site is clearly wrong.)
Partly the issue is that there are two positions for people to stand right up next to Jabba - on the actual platform of his movable throne (so on the same level as Jabba,) and on the stairs that are right next to it (so actually on a level below Jabba.) The stair step that people seem to stand on is maybe 6" lower than the platform, so it makes people look shorter next to him. And Jabba is almost always filmed from low angles, distorting how people appear next to him.
The old Jabba is undoubtedly about 1/2" too short. Until we get in-hand pics and comps with the new one, it's hard to say if it's too big or about right.
Jabba's a bit like Vader - I guess you want too tall, not too short. Though due to his body shape, the old SSC Jabba figure could probably have a 1/4" riser put under him without it being seen to raise him closer to the 11" mark
Here are the two clearest pics that show him in bright light and with someone clearly standing on his platform, not the stairs. If Bib was standing up straight (he's leaning over so he doesn't hit his head on the curved wall) Jabba looks like he would come up to a couple of inches below Bib's "human" chin, and Luke comes up to just under Bib's nose. Luke is around 5'-7" - that's how I calculated Jabba was about 5'-6" or just a bit under.
In this one, John Coppinger seems quite tall next to Jabba, though I don't know if Coppinger is tall like Michael Carter is - though in pics I've seen, Coppinger seems normal height next to people like Toby Philpott (he's a bit taller than Philpott.) If Coppinger stood up straight it seems Jabba would be below his chin.
