A circular loop is placed in an external magnetic field. How is the torque $𝞽$ related to the radius r of the loop?

The torque experienced by a current-carrying loop placed is the vector product of the magnetic moment and the magnetic field.

$𝞽$=m x B

And magnetic moment , m= IA

Therefore, torque acting on the loop, 

$𝞽$=IB(${\mathrm{\pi r}}^2$)

Hence, $𝞽$$\alpha$r²