It's terribly tempting to buy all the teeniest things because they're SO cute!!!
It's best to stock up on 0-3 month things. The little shirts will be baggy on scrawny newborn arms, then length may be a bit too long, but they grow so terribly fast, those newborn things will be obsolete within a week. A lot of clothing has the weight suggested on the label. I don't buy anything for under 8 pounds. My son was 8lb 12oz when born. Newborn fit only on the five days in the hospital. By the time we went home, we needed the next size up.
I actually felt like I got off easy with an 8 pounder. When I was born, I didn't get to wear newborn clothes. I weighed 11 pounds 12 ounces. My poor mom is only 5'9". Needless to say, I was her last. Ha!