1 – Neck
You should be able to fit at least two fingers vertically between the top of your dog’s neck and the harness for a medium sized dog (size M). A bit less for smaller dogs, more for bigger dogs. If using the harness for pulling, the neck opening should be quite snug to allow your dog’s shoulders to move freely.
Fingers on top of each other, between your dog’s neck and the harness!
2 - Chest
For medium sized dogs, you should be able to fit at least two fingers vertically between your dog’s chest and the harness. Adjust as even as possible on both sides. After initial adjustment for your dog, lock the chest straps by simply threading the end of the strap back through the sliplock.
3 - Armpits
Make sure there is room for your dog to move freely.
If you find it hard to take the harness on/off, twist it sideways when pulling it over your dog’s head.