Ehhh, I guess they are medium. They're definitely not large. It depend how I move, but mostly when I'm doing a lot of moving causes my straps to slip down.
I'm not sure if having the right bra size is the only way to prevent this. I think kind of bra it is,plays a whole. I've been measured, even measured myself , and I get the bra according to that size. Sometimes some bras sizes run differently. Like I can buy two separate bras from Rue 21, one will fit great and the other one not so much. Also how new or old (worn out) the bra is.
I have broad shoulders for someone my size but I'm tiny so they're actually small? Lol.
In any case, it's not so much about the right size bra as making sure the straps are snug. Tightening the straps makes the bra fit better anyway. You can have the right size bra (band/cup size) but if you don't make sure the straps are fitted correctly any bra will have its straps falling off your shoulders.
Thanks to swimming for the past 8 yrs, my shoulders are fairly broad, but bra straps still slip down unless they are extremely tight which subsequently hurts my shoulders. I just have to deal with fixing the straps whenever the fall down.
I guess maybe medium to medium small shoulders. I think it depend on the day, like if the strap gets a little loose due to activity, and if I am able to readjust, on an average day probably at least a couple times.
I wear the right bra size but because my damn shoulders are narrow that they always fall off if they're a comfortable size. So it's either really tighten my straps and stop my blood circulation or the right size and fix it back in place every 5 minutes.