Well, if you want to save a dog from the pound/Humane Society, look for a terrier mix. The size of the paws will give you a good indication of how big the dog will be, and yes, mutts are hardier.
Pure bred dogs have the advantage of being relatively predictable in temperment/size/health concerns, etc.
I did have a Corgi mix once... loved her... great dog.. but stubborn as piss!
Beagles are hard to train, I'd steer clear.
I'd go with a terrier or Schnauzer or a mixed breed that obviously has a lot of these breeds in it.
How much time do you want to put into training the dog? Golden retrievers are easy to train because they WANT to please. Jack Russells are extremely smart and will challenge you... foxy little things!
Keep us posted!