The pig and sheep are the same size as the new LEGO dogs (from the Police and Harry Potetr sets) and the cows are the same size as the standard horses. You can see more pictures and reviews, along with comparison to other LEGO animals at Brickzone's great site (home of the excellent Fabuland Housewife's!)
I think that minifig scale farms and zoo's are something that LEGO really should be making. There is a huge scope for sets with a very wide range of appeal. Just image minifig scale tractors, combine harvesters etc etc . LEGO are you reading this ... oh, you're not!