In the example you give the only place that the word "must" appears is in the statement "must be of medium size". I believe that referring to a vague statement "Medium size" , in the Non-Sporting group that would be the medium of 10 lbs to 150lbs or so, with the word "must" is almost equal to using the word "should"
Bruce Fisher
FISHERIDGE BULLDOGS
Respect the Elders, Teach the Young , Cooperate with the pack, play when you can , hunt when you must, rest in between, share your Affection, voice your Feelings, Leave your Mark!!!