Usually low weights with a lot of reps are used for this. Of course, since your main hormone is T for now, you will gain muscle just as any guy would. From my experience, it can sometimes be hard to have a gym instructor understand that, besides looking like a male, you don't want to gain muscle. Some people just assume you do.
What I used to do, is I told people I wanted to train to be in really good shape, but didn't care about muscle workouts and said very clearly I didn't enjoy lifting weiights. They never gave me high reps exercises for my arms, but it was mostly because I didn't ask to. We did a lot of aerobic stuff too. I did a lot of boxing, and though boxing training does gives you mass, the conditioning for the sport per se helps you stay lean as well.
With this said, I did a bit of gym hopping until I found a place that understood me.
Also, keep in mind, if you work out a lot as a guy, chances are you will look toned. The muscles may not be big, but they will show. It is just how it works for men.
On another hand, if you want to slim your arms, I think the best way to go about it is to not do any local exercises at all for them and lose weight. Do a lot of cardio, don't eat much protein, and you will slowly lose muscle. Once you are on HRT and given time, you will lose muscle faster, so IMO you should not try too hard right now.
