Just received my Turtle 2 air vest. Size Large is perfect I'd say for anyone between 5'5 to perhaps 6ft and 40 to 43 inch chest. If you're over 6ft and between say a 43 and 45 chest, you'll need the XL. I'm a 42 inch chest and can just get a fist between outside of my usual riding jacket and inside of air vest (to allow room for inflation). Length is spot on and it completely protects the whole back and up to just below where the helmet edge would be. In the event of deployment it inflates to provide a neck brace as well as full spinal protection and torso protection.
It's not too heavy either, and seems very well made. Lanyard supplied is quite short but there's maybe just enough give in the elasticated part to dismount without accidentally setting the airbag off. I'll be attaching that to the frame between petrol tank and front seat somewhere. Has to be the brightest dayglow yellow I've seen (shades recommended!). Small zipped pocket on chest, which some moan about, but large enough for bank cards and licence. I was expecting to look like a turtle with it fitted but it's surprisingly unobtrusive. No need for the back protector in the riding jacket either, as the air vest comes with a level 2 protector (over the air bladders) as standard. Top marks to Helite, seems to be a great product.