First, measure your dog's girth while the dog is standing.

The best way to get the correct size is to measure your pet. A soft tape measure is easiest. A piece of string or a belt can also be used and then measured against a ruler, or even estimated against a
known length, like a sheet of paper (A4 or A5.)
'Add 2 inches' is not an accurate way to estimate the size needed. Two inches is a lot to a small dog, but may not be enough for a big dog.
Measure snug against the fur, then add about 5% more to the total.
Keep it simple by rounding the initial measurement up to the next whole number if it is over 1/2", or round down if it is less than 1/2".
Estimating Example: Pet’s actual inches around the girth = 22".
10% of 22" would be 2.2". Half of 2.2" is 1.1" (or 5% of the total inches).
22" girth + 1.1" = about 23" for the adjusted girth length of the harness.
Next, choose the width of the material:

1/2" wide for small dogs up to 20 lbs (approx 10kg)
3/4" wide is safe and comfortable for many sizes of dogs.
1" wide is sometimes preferred for being more visible and can also spread pressure over a wider area, so it may be more comfortable for a hard puller. It can be bulky on smaller dogs, especially when adjusted to its smallest size.
Now that you have a size and width, you're ready to shop!
We make two types of non-restrictive dog harnesses:
Step In: Wrap-around style. Three adjustment points.
To use, unbuckle; put a front paw on either side of the center strap; bring the two buckle parts together over the dog's back and clasp together. Clip your leash to both D-rings together, over the buckle.
Adjustable Roman: Traditional over-the-head style is harder to escape from. Four adjustment points for more flexibility in sizing. To use, unbuckle girth; place the shoulder straps over the head; lift the left front paw over the center strap; clip the girth closed. Three options for attaching your leash: it can be clipped to either the O-ring on the front of the chest or over the shoulders, or to the D-ring on the tail-end of the top strap.