Baby size at birth

I’m just wondering is it true where they say you give birth to babies your fit for making? I’m just thinking because I’m really small down there like the opening area because when I was born my mom told me that my vagina was actually completely closed and now the opening isn’t wide kinda hard to explain and the guy I’m with he’s not big at all within girth. The reason for asking this is because people think I shouldn’t be buying newborn clothes because idk how big hes gonna be, I was only 6lbs and my bd was only 4lbs. And my mom has giving birth to all 6 and 5lb babies. So I’m just wondering because im already nervous about giving birth Nd being in so much pain. This is my first child so no need to judge, I’m not talking about the inside of my vagina I’m talking bout the entrance, like I explained when I was born I didn’t even have a vagina entrance 🤦🏼‍♀️ Jesus .