I didn't measure circumference because I'm more interested in knowing the actual width. It seems to be around 18-20 inches or 45.72-50.8 cm...

I'm only 5' 8" so I think my shoulders look extra wide because of my lack of proportional height. This post makes me feel dysphoric too. I even see cis guys with tiny frames, and envy them. lol!
But an interesting note, at least to me, is that my sister has broader shoulders for a cisgirl (16-17 inches) and she looks totally fine! Could be the boobs and hips tho.. XD
I'm not sure who we got our shoulders from, our parents definitely don't have very broad ones.