Reading and writing these values from Python must happen in *Object* mode. I added a plane and deleted one vertex, added the skin modifier and decreased the radius of one vertex.

    >>> obj = bpy.data.objects['Plane']
    >>> for v in obj.data.skin_vertices[0].data:
    ...     print(v.radius[:])
    ...     
    (0.25, 0.25)
    (0.25, 0.25)
    (0.08836718648672104, 0.08836718648672104)

Each element in `skin_vertices[0].data` has `radius, use_loose, use_root` to read and write. Radii can be any two element iterable (tuple, list).

    import bpy
    
    obj = bpy.data.objects['Plane']
    for v in obj.data.skin_vertices[0].data:
        v.radius = 0.2, 1.2

I just realized that <kbd>Ctrl</kbd><kbd>A</kbd> lets you constrain the effect on radius per axis with <kbd>Shift</kbd><kbd>X</kbd>,<kbd>Y</kbd>.