It really depends on the dog. You should really try someone else's or at least contact the company (J&J or Max200) to find out how to measure (most use a wooden dowel for this). I bought a "standard Lab size" for Murray and it is not working out for us (fine for training...not fine for competition). He has rather large flews so he actually needs more space between the ends and a thicker diameter bar to stop his mouthing. A recent judge (who happens to make articles) told me that Murray mouths the articles because they don't fit him well. I'll be ordering another set this summer.