What is the best GPS device for small dogs?

Quick answer:

As a general rule, all Tracker GPS devices fit for all hunting dogs, for none of them is that heavy that its weight would be a problem and there's always an option to attach the GPS device to a vest (instead of using a collar)

Respective weights for each device + collar + antenna

290g Artemis
242g Supra
222g G1000

More information:

Tracker Artemis in terms of design and shape is the best suited ergonomically for dogs, with no sharp edges and comes as close as possible to the dog’s neck as possible, so it's not hitting on dog’s chest or legs

Also, there's an option to attach the Artemis, not to a collar, but to a vest. 

This might make it easier for small dogs to carry the GPS device e.g. using this one:

Kardog Jehu 

A Hig viz safety vest with buoyancy aid (floats in water)

This Kardog Jehu vest is compatible with Tracker Artemis

For further information, please contact your local (or nearest) Tracker Retailer