From the outset let me say, you should have done this before you bought her, but now you have, so here goes.
Start with google, you bought a rather common spider and there are millions of articles and care sheets on this species. There is nothing to be petrified about. Unless you neglect her completely, she will not go anywhere. She must have any form of water supply without drowning her in a 5 litre waterbowl, food once a week, relative temperatures, no fiddling or useless handling and your admiration, and she will flourish.
Concern yourself more with her care than her size, that has no significance at the moment. But as a matter of interest, you measure from the tip of left front leg to the tip of right rear leg OR visa versa. Do this when she sits on the side of her enclosure and be aware it's always an estimate, else you need a vernier.
PM me if you have questions.