The need to include an intrinsic length scale in modelling elastoplastic material response at small dimensional scales has been commonly inferred over the past decades. Since then, many concepts have been stipulated as relevant in various scales and in different scenarios, e.g., geometrically necessary dislocations (GDN) and related pile-up effects, dislocation source limitation, surface effects, and other mechanisms. It is rather clear that it is impossible to reproduce the complex behaviour of the material over a wide range of dimensions by referring to only one mechanism. In the current work, we present a model of an elastic-plastic single crystal where two length scales are involved. The first one is related to the GND's formation effect that contributes to isotropic strain hardening. This part of the model involves a natural length scale that evolves during plastic deformation and is fully determined in terms of standard parameters [1]. It is shown that such an extension of the classical crystal plasticity theory is sufficient to predict macroscopic response during spherical indentation at the micro scale. The second length scale is related to lattice curvature and rotation effects introduced into the model through the Cosserat framework [2]. That approach is very convenient and computationally efficient since it involves only three additional global unknowns in 3D simulations. The influence of the lattice curvature and associated length scale in the modelling of the size effect in indentation is indicated.