Answer
$9.9mm$
Work Step by Step
We can find the radius of a proton's orbit as
$r=\frac{mv}{eB}$
We plug in the known values to obtain:
$r=\frac{(1.673\times 10^{-27})(6.27\times 10^5)}{(1.6\times 10^{-19})(0.66)}$
$r=9.9\times 10^{-3}m=9.9mm$