I,m,o, 29 is too big,I would not want a dog over 26 for any job with the exeption of maybe deer. A wagon can't slow down and change direction as easy as a mini,the strength to weight ratio is higher in a smaller dog, and a smaller dog gets up to speed faster. I've seen whippets easily beat greyhounds over 150mts. A realy big dog hasn't got the stamina of a smaller dog. Having said all that, a tall lightly built dog will perform better than a heavily built one. So I guess it's more about weight than hight.