How huge would a Martian colony have to be to be self-sustaining? They would need not only agriculture but also manufacturing of everything the colony needs, extraction of the necessary resources for they manufacturing, an education system capable of training workers to do all necessary tasks, a healthcare system (with all necessary support chains) and probably also an organised and stable system of government.

Just sending a handful of dudes to grow a little lettuce for a year doesn't make it self-sustaining.