The radius of a Bohr orbit is given by $r_n \propto n^2/Z$. For the same orbit (n is constant), the radius is inversely proportional to the atomic number Z. For H, Z=1. For He+, Z=2. Therefore, the new radius will be r/2. Note: The question states $He^{2+}$ which has no electrons, it should be $He^+$.