Meg has a Paws Aboard life jacket in XXS. She's 4 lbs. It adjusts to fit with velcro and two buckles under the belly and a velcro/buckle in front of the neck. We haven't had opportunity to use it yet - the water is still too cold, but I'm sure it will come in handy this summer.
Pink Polka Dot Doggie Life Jacket by Paws Aboard Inc at SouthernAgriculture.com