I think you’re going to want to be very careful when including sexism, even casual sexism into a narrative especially when violence is going to be an important part of that narrative. The attitude that comes from underestimating smaller characters or female ones isn’t one based in physiology, but psychological and, more importantly, societal attitudes.
These attitudes will not necessarily carry over when dealing with professionals. When working with the characters surrounding her, you’re going to need to remember two very important points: men are not stupid and female fighters are not rare, special creatures that will be ignored on basis of gender or size. Her cover will be blown the minute she starts fighting and will probably be given away long before then on the basis of body type, walk, the way she holds herself, and her movement pattern. Once that cover is blown, if the society she exists in is indeed much more hard lined and patriarchal, she’ll be regarded with a great deal more suspicion.
Professional warriors are a completely different animal than a non-professional one, they might overlook her in the beginning but when they turn around it will be with the same hard intensity that they use to treat everyone else. Unless she’s actively killing every man she comes across, her “secret” won’t be a secret for very long and if she is very good, then she will establish a reputation much more quickly than a male simply because she’ll be easier to pick out. Her position will be precarious.
Some really good reference material for this is: The Song of the Lioness Quartet by Tamora Pierce, Protector of the Small series again by Tamora Pierce, The Soprano Sorceress series by L.E. Modesitt Jr.
Below are the materials we wrote up dealing with this issue. You may also want to remember that for female fighters in patriarchal societies the dangers that they face are much higher than their male counterparts and the force they use will usually match that. They are often much harder than the men, and much more willing to take the force further instead of less because more force is often required to be convincing.
What would be the best way to portray a character who uses her smaller size to her advantage when it comes to fights? She is well trained and knows her stuff but she is often underestimated by anyone who doesn’t know her in the world she lives in because she’s small and female.