The radius of stars

I've been given some work on the radius of stars, by working out the speed and distance traveled of my first star I was able to work that one out. For the second star I have only been given its absolute visual magnitude (4.85) and it's velocity (188 km s) and the distance to the star (12.82 parsecs) , I'm pretty certain I need a formula to find out the radius from the distance of the star and it's absolute visual magnitude however I have no idea how to go about it!