My dog is a rescue mix, between 14-15 pounds. Ordered a small hoping it would work and it was perfect. My dog has typically hated being in carriers when we travel so I wasn't sure how he would like it but I think being able to face forward out in the open made a big difference. He seemed super comfortable when we tried it out. I was too! We haven't really gotten to use it for a hike or anything yet, but I am so glad we have it!

I got a medium bag for my 22 pound corgi, and it's a perfect fit for her long body. She was weird about it at first, but now it's where she wants to be during thunder storms, too! We got this because we're living in a city which requires that dogs be in bags on public transit. The extra storage space makes this a far better choice than other dog carriers.
The only "complaint" is that one of the buckles was a teeny bit melted/bent. It doesn't effect how the bag works in any way, and I think that it is the reason my bag was in the refurbished section. Again, it's not really a complaint, more of an observation.
I'd suggest this bag to anyone.

I ordered the Sport Sack Plus Large. I thought I would need a bolster ordering a Large based on the measurements, but after two uses, it is way to small and no bolster needed. I will be ordering a new one and hopefully be able to return this sack being to small and the bolster I didn't use. These are amazing, My dog was calm getting into it on the first try and I had no problem when riding with him. It was an enjoyable riding with him and the one little lick on my cheek told me he was happy and was not in any discomfort when getting the free ride. I had also ordered the goggles for him and they were very cheap and the lens kept popping out the couple time I would put on Jameson while round the house to get him used to them - When popped out the edge of the lens is very sharp and concerned me on safety for his eyes. I won't be using them (I am only giving 4 Stars simply because of the googles but the Sack is a definitely 5 Start)