It is not a stupid question but is also a question that depends on the individual. My experiences as someone who was tall and skinny with next to no fat were that I did not see real effects of HRT until I had actually put some weight on. At the time, I tended to range between 119-125lbs...
Now, that being said, there is also the element of determining whether surgery is in your eventual plans, as I found that being in good physical shape made the recovery MUCH easier (to the point of moving around effortlessly as soon as they let me out of the bed) and without the use of most post-op pain meds. And in that regard, it never hurts to be on an exercise regimen that works at toning certain parts of the body...