We perform the theoretical analysis of two wires of different size, simulating porous Si, through the linear muffin tin orbitals method in the atomic sphere approximation. We consider free, partially and totally H-covered [001] Si quantum wires with rectangular cross section. We show that (a) quantum confinement originates the opening of the LDA gap; (b) this opening is asymmetric: 1/3 of the widening is in the valence band, while 2/3 in the conduction band; (c) the near band gap states originate from Si atoms located at the center of the wire; (d) the confinement is enhanced in the case of free surfaces; (e) the imaginary part of the dielectric function shows a low energy side structure strongly anisotropic, identified as responsible of the luminescence transition; (f) the presence of dangling bonds destroys the luminescent properties.