That is assuming that the R2 is a normal R2 unit, but as we have seen in the Naboo N-1 Starfighter, R2 units can be modified to fit where needed while looking normal. If you compare it to the fully grown soldier fighting alongside it the height is different. As for the canisters on the shoulder they can be either seismic charges or oversized cylinder-shaped proton grenades like those used by the Rebels in the films, these are either thrown like massive mech grenades or manually loaded into the right shoulder tube and fired like a mortar or a missile. The primary firearm is a combination gun with the lower, round barrel firing actual projectiles, being for breeching or bypassing shields and the upper rectangular barrel acting like a powerful sub-turbo laser or gauss rifle. Extra ammunition for both barrels is stored in the boxes stored on the legs and on the torso under the arms. However, that is just what I have been able to find about the image, I am not Fan Zhang and he didn’t answer my email full of questions, so I am just going off of the consensus of the group that showed me the image.
