Well as far as how big can you get? That all depends on diet and what side affects your willing to deal with. You can get to be a 220 lb hulking beast, but your beard will put other guys to shame, your hair if you still have any will be super thin, your clit will be in competition with guys penises for who has the largest, and your voice will be as deep as Josh Turner.
Now if you're just looking for a little boost then I would say Anavar is the route to go. 90% of women can take 10 mg of Anavar with a 6 week on 6 week off cycle without having to deal with masculine features coming out, unless your one of these people who classify muscles as masculine features. Most likely though during the cycle you will feel an increased sex drive and some clit sensitivity.
If you are going with Anavar and not getting it with a doctor's script, you will need to get a home test kit and test whatever you get to make sure it is Anavar. Anavar is often times subbed with cheaper steroids like D-bol and if you take 10 mg of D-bol a day you will start having masculine features. Also make sure to order 10 mg pills not 40 mg pills.
When you start for the first couple days you will only take half a pill or 5 mg of Anavar as you feel out if you have any adverse side effects. After that you can jump up to half a pill 2 times a day at about 12 hours apart. This will put you at the 10 mg dose a day you want to be at. Try that with a 6 week on 6 week off cycle a couple of times then we can see if you want more or your liking the results.
Do not listen to anyone telling you to only take 2.5 mg of Anavar a day. You might as well flush the pill down the toilet once you figure out how much gets absorbed. Those 2.5 Anavar prescribers have no experience with how Anavar effects a woman's body.
Also you will want to take a multi vitamin while on the Anavar. As an ingestible it will hit your liver. At this low of a dose it should not hurt your liver, but why chance it, take something for liver support while on cycle as well.