Is the Montbell Long SS DH long enough?

I'm 6'1" and have a large frame (300#). I would think this to be spacious enough for your 6'4" height. I just picked up a #4 and it's got plenty of room to spare for both length and girth. There's even enough room to bend my knees and pull the footbox back into the bag (effectively making it a regular instead of long if you have shorter people using it). I've only used it a few times, but slept comfortably each time. I've used it down into the low 40s, so I'm not sure how it compares at or below its rating, but I slept in boxers and a t-shirt and was still warm inside. The super stretch design has finally taken larger people into consideration when making outdoor gear. Many thanks to MontBell.
Chris

I have the older model and I'm around 6'5". I find that it just barely fits me. I think 6'4" is accurate, but I make it work. Of course, it's only an issue when I'm sleeping on my back, which I rarely do. Are you a back sleeper or side sleeper? I think you'll be fine, it just compresses the insulation a tad on the top and bottom.

Side sleeper going quasi fetal sometimes so probably no.