I'm looking for a life jacket, too. The little one I have now doesn't have a handle on the back and is awkward.
BUT this one
http://www.amazon.com/Doggles-XX-SMALL-Flotation-Jacket
actually has a great size chart that looks like will give the best fit. Check out the chart. Silly that the photo of it is of a big dog.
I'm going to use the little one I have til I see how big Maxwell will end up.
I used a wicker bike basket where I could secure him in with a harness. Now that I have a livelier puppy, I may have to rethink that and find the one like a cage on top.
Love to hear back what you decide on.