Small Stature means a character is at the minimum for whatever career path they've chosen (or have figured out how to cheat the system that would otherwise bar them). In my personal experience, I've met some people in the military who I would swear were too short to be allowed to serve, but there they were! Stats below...
Small Stature (100 XP)
Characters of Small Stature take no penalty from units with the Small Cockpit quirk (which is a -2 quirk). Their small size seems perfectly fitted to those units. Their smaller than average size means they're generally not as strong as those of full height (leverage counts), so their maximum Attributes for BOD and STR are limited to 6 (they can achieve 7 with the Exceptional Attribute trait). They can use "normal" cockpits with technical adjustments that only need to be made once to any individual machine. If a character with Small Stature uses a normal cockpit without adjustments (for some reason), they take the same penalties for Piloting imposed by the Small Cockpit quirk.
While all children technically have Small Stature, their general lack of skills prevents them from meaningfully benefiting from this quirk.