PetSafe does sell a little dog electric fence. Not sure who else does.
PetSafe PIG00-10773 Little Dog In-Ground Fence with Collar - Electric Dog Fences
Cozy my mama dog is + 16 pounds but is trained to a wireless electric fence by PetSafe when she was 10 pounds. The company stated over 7 pounds which she was and still is. She is fine with it. Never bothered her. I doesn't use it at my parents house though anymore because she harresses their new neighbor who is afraid of dogs so leash for her there.
Everyday is different. I know she is the only yorkie of mine who is appropriate for it. Roxy and Lucy are very obedient. Zoey is a runner but is a timid personality.