Maybe there's a breeder on this board who can help you more than me, but I wanted to second Steltz's comment while adding my 2 cents.
My 2 cents: Are you going to be participating in conformance/dog shows? Usually the breeder will already have pick of the litter, especially if you are just buying your pup for a companion. IF there is a remote possibility you want to show your dog (in conformation), then I would have serious discussions with your breeder as they like to be involved and can help out immensely, and might end up wanting to co-own the dog with you (though this is always not the case) especially if the breeder wants to keep pup as breed stock. but I am getting off topic!
IF you're buying your pup to hunt with, there might be some other things to consider. Obviously Vizslas are naturally full of vhem and vigour, but are the parents successful hunters with lots of energy, drive and determination? Can they run all day long? A lot of breeders NAVHDA hunt test their dogs for ability - if so, the dog would have title after their name like JH or MH after successful competitions.
There is a link to vizsla breed standard here:
http://clubs.akc.org/vizsla/standard.htm which might help you determine some physical characteristics to look for.
Steltz is right: "the runt may become the biggest in the pack while the one with the wide head may have the skinniest head as an adult". And most importantly, "I would pick the one with the personality that fits yours best". There's also "personality tests" that can be done on a puppy that are supposed to predict a dog's mannerism when they get older (try a google search). Our pup was the largest of the litter and is growing up to be fairly thin and lanky. But he's 9 months and we can't seem to be keeping up with his appetite!
I would pick out a pup on personality over looks any day (unless you are showing). They should be with you upwards of 15 years; so personality and temperament count!
And health of breeding stock should also take priority. Usually, the parents should be at least two years old and have OFA hip certifications of at least "good."
Well, that was a lot more rambling then necessary and didn't really help you determine how to pick out physical characteristics. But good luck to you. Our Vizsla is the best dog I've ever had.
