After some sleuthing I managed to track down the author (Phil Geusz) on his IRC channel and posed him this question. I’m sure he won’t mind me sharing his response;
Q. I was wondering how tall rabbits are in the David Birkenhead stories. In feet and inches
A. Hrrm. I very carefully never gave a specific figure. It's much easier to write that way. But David was shorter than most. And all were shorter than human-normal. In my mind, they're about the size of a 13-year-old. (But I may well have contradicted that[!])
Anthrochat: #TheZoo – 13/10/18
His intentionally vague description suggests that David is somewhere around 4 foot 5 inches tall (slightly shorter than the average size of a thirteen year old boy) which in turn lines up quite nicely with the best description in the novel, indicating that he’s the shortest, by a pretty wide margin, than the shortest human out of several dozen volunteers, but not so abnormally short that he can’t function in his unit.
"Because he didn’t specify if he meant his right or our right—quite deliberately, I suspected—there was even more confusion. But finally we formed a ragged, ill-kempt line. “Well, young gentlemen!” he declared with his foghorn voice. “That only took maybe ten times as long as it should’ve.” He strode down to my end of the line—I was easily the shortest in the whole class, not just the squad. “Name?” he demanded, lip curled in disgust."