Jim,
I would not be too alarmed at the smaller size. Russ is right in what he says in his post below. Thayeri can be fussy eaters to start and really don't take off until they are eating at least medium fuzzy mice. I have over 30 here right now and my adults range between 35 and 42 inches. I have 2002s - some that I produced and some from other breeders - that range in size from 17 to 26 inches. In addition to the fact they they may start out reluctant to feed on mice, some breeders brumate all of their snakes during the winter, another factor in the smaller size.
I would be more concerned with the overall health of the snake - is its girth proportional to its lenght? You don't want an emaciated snake, nor do you want a real porker.
Most of my thayeri have taken longer to reach breeding size than my other kings, but they are healthy and produce clutches of 10 to 16 eggs. They are a rewarding species to work with and are definately worth the extra time needed to get them to adult size. Best of luck wuth yours.
Bob Johnson

Reptile Artistry