I'd recommend that you buy it. I got the similar 'Blue Kazoo' bag in 1975 and it's been used on multiple trips every year since then - most recently on a trip this last weekend. With the addition of a couple ounce vapor-barrier liner it has kept me comfortable on nights that got down to about 0F while stuffing down to a size that just uses a small fraction of a small pannier.

As to the size it's hard to say without actually getting in and trying it out. I was faced with the same issue - there was only one of the 'regular' size still available on sale and nominally I would have needed the 'large' size. So I got in to check and there were a couple other shoppers waiting to see if I'd decide it was too small. I'm glad I got it since I've been very happy with its performance and comfort for the last 42 years.