I guessed at the size I needed and ordered the 'Medium' on Saturday night and a 'Large' has just been delivered! (Tuesday morning) so a really good turn around but the wrong size.
The straps are enormously long, why they need to be that long I don't know?? and even tightened all the way are still a 3-4 inches too long for my Bull cross (about the size of a fit retriever) but the pack is actually a nice length/size so I'll just modify the straps to fit.
First impressions are that it's very well made from sturdy, waterproof material and has decent double stitching so should last for a fair while.
One of the snap shut buckles was on the wrong way round but I've just slide the buckle off and turned it round.
For reference the pack came with a tag stating the sizes (something the advertising doesn't)
Size Pack length Dogs Girth (central strap length)
Giant 38cm (15") 75-150cm (29-60") - 60 INCHES!! that's roughly the size of a Horse!!!
Large 33cm (13") 60-130cm (23-52")
Medium 28cm (11") 50-95cm (19-38")
Small 24cm (9,5") 35-65cm (13-26")
Hope this helps.