The most probable radius (in $\text{pm}$) for finding the electron in ${\text{He}}^{+}$ is:

The most probable radius for a hydrogen-like atom is given by:
$$r_{mp}=\frac{a_0}{Z}$$
Where $a_0=52.9\text{pm}$ (Bohr radius) and $Z$ is the atomic number.For Helium ion (${\text{He}}^{+}$), $Z=2$.
$$r_{mp}=\frac{52.9\text{pm}}{2}=26.45\text{pm}\approx 26.5\text{pm}$$