Huh, I counted his tail in the estimation, but it seems it's not supposed to be counted.

Using Haley as the unit of measurement, it seems the body, the tail, and the neck are all very approximately 1 haley long.

Assuming that 1 haley = 5.5 ft as typical for human females, that makes the relevant measurement around 11 ft, which means Large, which would make him young adult/adult.

To be medium size, he would need to be only 1.5 haleys long, which seems less than the depicted length, though not by that much, so I guess it could indeed be juvenile as well without too much of a stretch.