I have an Avery vest for my GSP. Fit is easy to sort out if you take it to a wetsuit fitter. I couldn't find one in my area so I took it and my dog to a lady who does leather work for horses and she fit it for me. She tightened up the area around the neck, the waist, and the shoulders so it fits snugly all over with no bits hanging out. She also trimmed between the front legs and increased the clearance for his tackle. My dog has the typical GSP large shoulders and chest and narrow waste so as much or more fitting would be required for his as any dog.
It works really well. I don't use it all the time. Just first thing in the morning or late in the evening on really cold, windy days. It was cheap though so it's worth it to have on hand for the few days. I didn't do anything special with him before strapping it on, just took him out hunting. He didn't seem the least bit phased.