I'm fairly big (118kg) and only just fit today. There is a test seat that the entrance staff will let you use even while the queue is outside, just ask, I could get a green light but only with some pressure on the bar. On the actual ride, initally I had my restraint unlocked and had to reposition slightly sideways to allow more room, then it was fine. You can feel the pressure on your thighs though.
The only other ride that feels tight for me is Scooby, so I'd say Leviathan is not very forgiving compared to ST/rivals/superman etc., at least for me.