Good advice above: health guarantee, ask to see the parents, research the breeder's name on the internet. Ideally the breeder shouldn't let pups go before 12 weeks, but definitely not under 8 weeks.
A number of factors influence price: location, girls are usually more expensive than boys, champion sired and so on. I would be leery of a price too low, and one too high.
Ideal size depends on you and your lifestyle. I chose larger because I'm tall and my 9+ pound boys seem small to me.
Admittedly, they were easier to carry together at 6 - 7 pounds.
Above all else, the best recommendation is patience. Listen to your instincts. Best wishes in your search.