Trying to put Halti onto a wriggly strong willed pup I wish the instructions were easier deciphered. Just having the box insert the print is much too small and illustration tiny.
I hope I have it figured out so I can use without a struggle on my willful Cavalier.

Medium size too large and small size too small and lightweight. Was trying to fit a 'Toller' which a similar size to a border collie and about 20kgs.

Purchased Halti harness as last resort to control large pup. Due to urgent surgery at 12 weeks of age and essential crating post-operative 24/7 (except for toilet breaks) for 5 weeks following her procedure, the critical period of training unfortunately couldn't be completed during this time frame. Once able to commence training, she had grown significantly, was very strong and strong willed ... finding a harness / lead to assist with her training was difficult until stumbling across the Halti harness. For her, this has proven to be the most successful in controlling and assisting with her training ... certainly working for my husband and I, and our dog.