The thorough understanding of size effects in crystal plasticity is still a challenging task even though consent is reached in literature on the strength scaling behaviour in single crystalline face-centred cubic structures. However, for body-centred cubic metals, controversial results have been reported, which is mainly attributed to a temperature-dependent thermal stress component that significantly alters the strength scaling behaviour. An even more complicated situation is expected if additional interfaces, such as grain boundaries, affect the crystal plasticity at small scales. For technical applications, the understanding of small-scale plasticity of structures containing interfaces is crucial, as design optimizations may lead to further miniaturization of devices and lifetime and reliability issues might be enhanced. In this PhD thesis a possible influence of interfaces on the strength scaling behaviour of two body-centred cubic model metals, namely chromium and tungsten, is investigated. Macroscopic compression, nanoindentation and in-situ scanning electron microscope micro-compression experiments, spanning several length-scales and ranging from the (sub-)micron to the macroscopic regime, were performed. Furthermore, temperatures ranging from room temperature up to ~600°C were applied. To conduct experiments at small scale and elevated temperature, an existing micro-indenter was equipped with a custom-built heating device. For the first time, temperature calibration was performed by conducting a simulation approach to perform micromechanical tests up to 300°C. The apparent thermally activated deformation behaviour was addressed in terms of strain-rate sensitivity and activation volume. The influence of free surfaces was examined by thoroughly reducing the plastically deformed volume until a transition from bulk-like to single crystalline deformation behaviour was observed. The present work identifies contributions of microstructure and free surfaces to the strength scaling behaviour in body-centred cubic structures. Underlying deformation mechanisms in ultrafine-grained chromium and tungsten are examined and related to the single crystal situation. At low temperatures single crystalline and ultrafine-grained samples deform by the thermally activated motion of screw dislocations. At elevated temperature, the constant activation volume indicates dislocation-grain boundary interactions.