Spin-polarized electron beams are an essential tool for exploring new physics beyond the Standard Model, investigating magnetic properties of materials, and generating polarized photons and positrons. Here we put forward a method for the generation of ultrarelativistic electron beams with high spin polarization, where a linearly-polarized ultraintense laser pulse interacts with a nonprepolarized transverse-size-tailored solid target. The radiative spin polarization is facilitated by the asymmetrical standing wave formed at the overdense plasma surface. Strong electron heating caused by transverse instability enhances photon emission in the density spikes injected into the standing wave near the surface, where the field asymmetry creates substantial polarization. Two groups of electrons with opposite transverse polarization emerge which are angularly separated due to the in-phase oscillation of the magnetic field of the standing wave and its vector potential, and are promptly propelled into the plasma slab. Via a proper target design, the polarized electrons, as they exit the plasma, are focused by the self-generated quasistatic fields. Our particle-in-cell simulations demonstrate the feasibility of producing highly polarized electron beams with 10 PWclass lasers, e.g. with polarization of 60% and charge of 8 pC at energy of 200 MeV within 15 mrad angle and 10% energy spread. This simple scheme provides a robust way for highly spin-polarized relativistic electrons with ultrastrong lasers.
 

electron acceleration,spin polarization,laser plasma interaction