First, you may want to consider starting your own thread in the future. This is a very old thread, and it is usually considered rude to ask your own question in someone else's thread.
With that said, welcome to the forum! Do you mean that each betta has it's own tank, or that you have two in one tank? If you only have two in that tank, I would separate them immediately. Bettas are extremely territorial, even females, and they don't do well at all in pairs. Either that, or start a sorority with 4 or so more bettas, but this is not for the inexperienced fish keeper, or the faint of heart (even in a properly set up sorority, deaths can and do happen).
Females will end up being anywhere between 2" to 2 3/4" long. Some may be slightly shorter and some may be slightly longer, but most fall in this range.