Answer
$H^{-}$ is larger than $He$.
Work Step by Step
H consists of 1 proton and 1 electron.
$H^{-}$ is formed by the addition of one electron to the H atom. So it consists of 1 proton and 2 electrons.
He consists of 2 protons and 2 electrons.
The size of $H^{-}$ is larger than $He$, since 1 proton in $H^{-}$ less effectively attracts 2 electrons whereas in $He$ 2 protons attract 2 electrons more effectively.
