Here is the full ship, after assembly in Earth orbit. The components are sent up via various conventional launch vehicles. As such, the whole vehicle is never intended to withstand full Earth gravity. This saves considerable structural mass.
On the nose of the ship (the top in this picture), is the Mars Lander Module (MLM). It is attached upside-down to the rest of the ship, and has big round meteorite shields covering the bottom (leading) surface. The rest of the ship is the Command and Service Module. The top half of that is the Command section, housing the astronauts' living space. The bottom half is the Service section, with the engines and various bits of machinery such as life-support, gyros, and energy cells. Four external fuel tanks are also attached to the Service section.
The engines on the CSM may seem undersized, but they don't need to be large. On a journey where you have plenty of time, long slow burns are sufficient. And the smaller thrusters mean less weight. Less weight is good.
Two of the external tanks are exhausted on the outbound acceleration burn and ejected just before deceleration. The other two are used for the deceleration burn and Mars orbit capture, then ejected just before heading back to Earth.
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