A weapon of greeater then your size has to welded two handed. A weapon equal to your size can be welded with one or two hands. A weapon smaller then your size can only be welded with one hand.
So a medium creature has to weld a large we aponj in 2 hands. They can weld a medium weapon with one or two hands. And weaons that are small sized or smaller can only be welded in one hand.
As a side-note, when welding weapons, always be sure to wear goggles.
Safety first!
Apart from that, there's nothing actually stopping you using a light weapon two-handed. You just don't get the bonus damage from it.
A weapon of your size category or one size larger gets 1.5x your Str modifier to damage when used two-handed; a weapon smaller than your size category only gets 1x when used two-handed.