Size does not really matter!
At least for the legs.
It does not matter if you have slim legs or even large legs.
If you have the risk:
You could have the smallest legs and still can develop varicose veins.
I would argue that the smaller your legs are, the more obvious it is that you have varicose veins, as they quickly become visible.
The reverse is applicable. If you have larger legs, it may take a longer time for them to be evident on your skin.