It will definitely have an effect on the weight as 7 pounds is the highest standard for a yorkie and 10 pounds is over standard. that DOES NOT MEAN that a yorkie can't be over/under standard (like the really tiny ones that some try to label as "teacup"
). there are lots of yorkies who are different shapes and sizes, but if you're just looking for a loveable pet, then it's totally up to you.
my husband wanted a bigger yorkie anyway, and our derby was the biggest of the litter and no one wanted him
his parents were within standard, but on the bigger end of things, so it's likely that our derby may end up bigger. he's already 7 pounds now, and he's only 4 months old! we have heard that his siblings are smaller, but it didn't matter to us either way.
as far as health goes, you run into trouble in the very little ones. there are very common yorkie health problems, which i'm sure you've read (if not, please read) and other than that- you just have to hope for the best. there's no real way of knowing unless you know a full history of the bloodline of the parents, and even then there's no guarantee.
i love my derby, and i wouldn't care what size he is. some people have a preference for size, but there are yorkietalk members who bought yorkies from breeders who had standard sized parents and their yorkie still ended up way over standard. sometimes, it just happens. would it make you love them any less?
it's your baby.