in my opinion dave once you go over 27tts i think in 9 out of 10 cases you start to lose that little bit of agility sometimes required to pick up rabbits on flat land. on the rough land it wouldnt be noticed but when it comes to the flat land the dog might look as though it is lacking something. thats when decent bit of brain is required in a dog to make up for its slight running/agility disadvantage.
i prefer a bigger steadier dog because i think they can just take that little bit more when needed o.k they might not look has impressive as a lighter faster built dog on a rabbit but has lo