According to my wife (she has been an intervention specialist for 12 years, teaching at both the elementary and junior high levels) physical size should not play a huge factor in deciding whether or not to send your son to Kindergarten at age 5.
Kindergarten has really nothing to do with sports or being physically bigger, it has everything to do with being mentally ready. If you look into any kindergarten classroom you would see kids in various sizes but there was one constant, they were mentally ready.
With your son turning 5 next month, he will be one of the oldest in the class, and that does come with some advantages further down the road. You also need to look at his friends being in the same grade or if he doesn't go to kindergarten this year, will they always be one grade ahead ? That does affect them later on in junior high/high school.
Ask your school district for a paper that contains things he should know for kindergarten and if he knows all of that I suggest sending him. He might catch up some day in size or he may never be 6' or taller, so what then ? Will you look back and say, "Should we have sent him anyway ?"