This is something I wondered and thought about a LOT pre HRT -- this is my conclusion based on experience and conversations with others:
As Amy said, muscle mass will diminish but not bones. It's really that simple. If you were muscular to begin with, HRT will reduce muscle mass. In this case, apparent shoulder width may reduce because the larger muscles tend to hold bones further apart. So as the muscles shrink, so will the spacing between the shoulder and arm.
If you don't have significant muscle mass to begin with, you won't really see a difference in shoulders width. Muscles will still shrink, but the affect on bone spacing is almost non existent.
However, the growth of breasts will make a big difference in the appearance of your shoulders and frame in general. On a male (with a flat chest), there appears to be a lot of empty space between the shoulders, which exaggerates the look of broadness. By have something fill the space between the shoulders (breasts), the appearance of broadness is greatly reduced.
Your entire self image (mental as much as physical) changes with HRT, and there's no telling what you will and will not like about your appearance after change has become apparent. I personally was always very self conscious about shoulder width, but that changed completely as I acclimated to the HRT and got used to life as a female. I became completely comfortable with my frame and now my favorite thing to wear in the summer is a tank top.
Also I should point out that there are no surgeries to fix shoulder width -- you can just imagine how invasive something like that would be.
