I didn't really know my big that well when we chose our bigs last year, since she was a junior. But now we're really close, and I'm sad that she's graduating this year and I'll be bigless for two years (I'm a sophomore).
On the other hand, I know both of my littles really well (I have twins!!). I went to high school with one an dlived next door to the other last year (she's a sophomore, too, and decided to rush this year).
Either way, don't worry about it. Even if you don't know your big that well now, you're bound to grow closer. And don't forget...people *will* try to mislead you about who your big is!