Assume that the radius of the first Bohr orbit of hydrogen atom is...

For hydrogen like species, from Bohr's formulas we get,
$r ∝ \dfrac{{n}^{2}}{Z}$
where $n$ is orbit number, $Z$ is atomic number

Thus,

$⇒ r{He}^{*} = {r}_{H} \times \dfrac{{n}^{2}}{Z}$

$⇒ r{He}^{+} = 0 . 6 \times \dfrac{{\left(3\right)}^{2}}{2}$

$= 2 . 7 Å = 270 \times {10}^{- 12}m$

$∴ r{He}^{+} = 270pm$