Traits don’t always have to be personality-based:

You are Short. People may also take you less seriously for your diminutive stature, and you may have trouble reaching hand-holds when climbing, or striding across wide gaps. Yet you are also able to fit in spaces that others would not, your abilities may be underestimated by others, and you might avoid that trap set to slice at the heads of your taller companions.

Your Size will be in the range of (2 × your Short rating) lower than it would otherwise be. It may be slightly higher or lower, depending on how densely muscled you are.