Each trapper has his own opinions on the subject, but I prefer a #3 for coyotes. There is nothing wrong with a #2, I just like the wider kill area on the #3.
Couche's statement about the #3's on land makes little sense to me because the spring pressures aresimilar between the various sizes.The only difference would be the depth of the catch- you're more likely to have him standing there caught by the ankle instead of the toes.
My traps are different because I have modified them greatly... but I have modified #2 Bridgers in the same fashion and there is no difference in strength/speed/durability. Only the width of kill area when the trap is set.
The only thing worse than catching a dog is smashing his toes real good or having him take the trap home. The responsibility involved with trapping is very high, take it seriously no matter what target or non-target animals you're catching.
Here is a story from my trapline last year, pics included, of a situation I encountered with a domestic dog. Hopefully this helps;
http://midwestpredator.groupee.net/eve/forums/a/tpc/f/3371088811/m/8011027031