No, you can never have a cage that's too big. I have one rat in a double Ferret Nation right now, that's suitable for up to 10 rats. The more space, the better.
However like AMJ said, she will need a friend. The cage you buy should at least be suitable for two rats, preferably more. Two square feet each is the bare minimum for each rat, but a lot more is preferred. The cage calculators are only so accurate because they can't take into account extra levels, shelves, space taken up by toys, wasted space, etc. but they give a good estimate
