Anyone good at sizing stuff? What would 50'' bust be in a top?

[alltoomuchrightnow]

Selling a top that is labelled as Medium but it's definitely an oversized medium, as armpit to armpit measures 25 inches.
What does this roughly translate as if not wearing it as a deliberate oversize?
eg UK 22 , 24 etc

sorry don't know why this got posted twice. First time I tried to post it , a message came up saying 'bad gateway'.