The RADIUS of the H atom is about 0.5 angstroms.
Although the shape of the atom is spherical, assume it occupies a cube whose side is twice the radius. Find the volume of the cube, and multiply it by Avogadro's number.
V = L3A0 = [2(0.5 angstroms)]3 (6.02 x 1023) = ?
This gives volume in angstroms3, which you can convert to m3 using
1 angstrom = 10-10 m