Several months ago I had a Facebook follower tell me her dog would escape every harness she put him in. She would turn away and back within a few seconds and all that would be left was a leash and empty harness.
Granted some dogs seem to have a magical ability to escape all forms of confinement. A harness with an extra strap around the belly can help prevent your Houdini from getting loose.
Another issue to consider is whether the style of harness will rub under the front legs or restrict the movement of the shoulder.
The best way to measure your dog for a harness is to use a fabric seamstress tape or a length of string to measure around the dogs chest behind the front legs. (If using string, mark the string where it meets, then measure against a hard tape).
Here is a video from Pet Circle.cm.au on how to fit different types of harnesses:
https://www.youtube.com/watch?v=DeDcWpvtEVw
CAD $
