Labrador Retrievers are NOT small by any means - my female lab is 75 pounds, and she is not overweight at all. You realize that a puppy grows VERY quickly, right? I got my lab at 12 weeks old, she was less than 20 pounds, in another 3 months she was nearing 50 pounds. So it will not be a little puppy for but a couple of months.
Labs need a lot of exercise, and especially as puppies are super hyper and active. They are a sporting breed, and are bred to hunt - so again, daily exercise is a must for this breed.
Labs are also very social and bright dogs, and they are extremely affectionate and need a lot of attention. This doesn't mean you cannot leave the dog home for 8 hours at a time when it is an adult - my lab is fine for 8-9 hours a day, 5 days a week now, but until she was a year old, she would whine and bark when we left her in her crate when we went to work. If we let her out of the crate while we went to work/school, she would chew up furniture. But with age and some training, she grew out of it and is now well-behaved when home alone. However, I do take her for long daily walks, and aside from school/work we spend most time at home, and give her a whole lot of attention.
As I stated earlier, I would never buy a dog from a pet store. They are notorious for being inbred from puppy farms (if you don't know about the abhorrent and disgusting practices of puppy mills, do a little research). Everyone I know of who has bought a dog from a pet store has had major health problems within the first 3 years they had the dog. They are not bred well, and you are right - they often have weird temperament issues, whether abnormally timid or abnormally shy/aggressive. I would only buy from a breeder, where you can see BOTH the mother and father of the puppy, who has a proven track record of breeding healthy dogs, and you can see exactly where the puppies have grown up, and how they have been socialized, etc. Either that, or I would get it from a shelter, as has been stated many times.