These harnesses are a great functional design that doesn’t in anyway hurt, or impede your dog's movement.
 
There are 5 adjustments to ensure a good fit, and there are clips on the neck as well as chest so the harness doesn’t have to go over the head.
 
Approximate Size Guide
 
XXXS: 27-33 cm around chest (toy dogs, puppies small breed)
XXS: 30-45cm around chest (e.g. Chihuahua)
XS: 40-55cm around chest (e.g. Jack Russell terrier, Pug)
S: 50-70cm around chest (e.g. Beagle, Tibetan terrier)
M: 60-80cm around chest (e.g. Labrador, Golden Retriever)
L: 70cm-100cm around chest (e.g. Viszla, Pointer)
XL: 90cm-130cm around chest (e.g. Great Dane)
 
WHY A HARNESS?
The main reason I use a harnesss is to limit any possible damage a collar can do should my dog pull when on a lead, and a small yank because of seeing a cat or being spooked can do damage, as well as a dog generally pulling on the lead, a dogs neck is no different to ours AT ALL, I leave it to your imagination.
The neck is part of many functions in a dogs body,the nervous system, blood vessels, bone structure, thyroid glands sit in the neck, etc..... A dogs health can  be damaged in many ways through wearing a collar on lead .
When buying a harness there are two important things to look for.
The first is that the front is Y shaped,
So that should your dog pull the pressure is on the sternum (bone) not on the soft tissue of the neck, and that it in no ways impedes with the movement of a dogs front legs.
There are many different shaped harnesses on the market, some come across the front legs impeding movement, some tighten as a dog pulls hurting a dog under its front legs. Some sit as high at the front as a collar totally defeating the reason you're a harness.
The second is a harness should sit 2-4 cms(two fingers) behind the front leg so it doesn't chaff inside the dogs front legs.
It is also important that a harness doesn't go over the dogs head, many dogs dislike that sensation so two clips on the side makes your dogs  life much easier.