<p>Aiming at the obvious size effect, extremely weak elongation, and failure of traditional calculation models of microsized materials, polycrystalline (the ratio of the strip thickness <i>t</i> to the grain size <i>d</i> is &gt;4), multicrystalline (4⩽<i>t</i>/<i>d</i>&lt;1) and quasi-single-crystalline (<i>t</i>/<i>d</i>⩽1) materials with different grain sizes were designed using nickel-based ultrathin superalloy strips. In this study, we systematically investigated the strain-hardening behavior, the dislocation slip mechanism, the transformation of geometrically necessary dislocation strengthening. The size-effect threshold on the mechanical properties and deformation mechanism was successfully determined. At an approximate threshold <i>t</i>/<i>d</i> value of 4, the reduction in slip multiplicity and delay in cross slip led to material transform from a traditional polycrystalline to a multicrystalline state. The multicrystalline strip occupied continuously refined slip bands, and had outstanding work-hardening capacity and reduced void growth rate through the accurate manipulation of its intrinsic dimension and size effect. Therefore, it could overcome the trade-off between strength and plasticity, which improved simultaneously. The products of the strength and plasticity were 2.5 and 2.9 times higher than those of fine-grained (<i>t</i>/<i>d</i>=10.7) and coarse-grained (<i>t</i>/<i>d</i>=1) samples, respectively. This study improves the deformation theory and traditional formula model of microscale materials and provides novel guidance for preparing ultrathin strips with perfect strength-plasticity matching.</p>