How hard would it be to retrieve some samples from Mars?

A small rover, emerging from a landed module. Goes along the surface
and picks up say 50 kilos of samples. Then it returns to the module,
it takes off and comes back to Earth. Or, it could dispense the
samples, leave the return craft and stay on Mars doing more
experiments.
How large would such a platform need to be to escape Martian gravity
which is 1/3 that of Earth's?