Two fingers, laid flat, under any strap. That is the whole answer, and the rest of this is about where people apply it wrong.
A harness that is too loose fails visibly. It rotates, it rides up, the dog backs out of it in a parking lot. A harness that is too tight fails quietly, over weeks, as rubbing and restricted movement.
Check every strap on its own
Most harnesses have at least two adjustment points, and people set one and assume the rest followed. They did not.
Test the neck strap and the girth strap separately. Slide two fingers flat, palm against the dog rather than knuckles, under each one in turn. Both should pass with light resistance.
It is common to end up with a neck strap that is correct and a girth strap two inches loose, or the reverse.
Too tight
The signs, roughly in the order you will notice them:
- Hair thinning or breaking along a strap line
- Redness or chafing where a strap crosses the armpit
- The front stride shortening, so the dog takes smaller steps than usual
- Reluctance to have the harness put on at all
The armpit is where a tight harness does damage first. The skin there is thin and the strap moves across it with every stride.
Too loose
- The harness rotates until the back attachment sits on the dog's side
- The chest strap creeps upward into the base of the throat
- You can lift the harness a hand's width off the dog's back
- The dog can reverse out of it
The last one is the dangerous one. A dog that has backed out of a harness once has learned that it can, and it will try again at the worst possible moment.
The shoulder problem almost nobody checks
If the chest strap crosses over the point of the shoulder, it limits how far the front leg can extend.
On a walk around the block you will not see it. Over miles, it changes the dog's gait.
Watch your dog walk away from you wearing it. The front legs should reach the same as they do without it. If the stride looks clipped, the strap is sitting too far forward, and that is usually a geometry problem rather than a tension one.
It moves after the first few wears
Webbing settles and buckles creep. A harness set perfectly in the living room will not be set perfectly after three walks.
Recheck the fit after the first week, then periodically. Monthly is plenty for an adult dog, more often for a puppy or any dog whose weight is changing.
What tightening will not fix
A harness cannot be tightened into fitting.
If a dog backs out of a correctly sized harness, the answer is a different design, usually one with more adjustment points or a different chest geometry. Cranking the straps down to stop the escape gets you a dog that cannot escape and cannot move properly either.
The same goes for rotation. A harness that spins at a correct tension has too few contact points for that body shape, and that is not something you can adjust your way out of.
When you are comparing harnesses, the number of independent adjustment points deserves more attention than it usually gets, because it is what decides whether an awkwardly built dog can be fitted at all. Ours are in Tactical & Trail. If you have not taken your dog's measurements yet, start with the rest of the Field Guide.