Rationally, this seems to me like something that comes with experience (knowing how much force to use with a given partner). I still worry about it though, because I don't want her to feel like I'm judging her strength due to gender (if I am using too little force).
Kudos to the original poster for being sensitive enough to worry about the issue but my two cents (being a small woman myself) is that you are highly unlikely to come across as patronizing. It is a fact of life that there are general correlations between size and strength as well as between gender and strength. I'm a lot stronger than most women my size but I don't get particularly offended if people assume otherwise...I just enjoy surprising them.