Designing PCBs to handle Mains
It's not about wire size and trace width.  Those are dictated by the current no differently than for low voltage lines.  The issues is insulation and spacing.

To be safe, leave 5mm creapage distance between anything line-connected and everything else.  There are various standards, but if you can keep to 5mm spacing, you're pretty much covered.

I can hear Russell already typing in New Zealand warning you in capital letters and italics how dangerous anything connected to the line is.  Yeah, yeah, he's right but I'm figuring you know not to stick your finger in a light socket and put the other hand on the kitchen faucet.  Besides, if you want to kill yourself that's your business.  That's why we keep a supply of Darwin awards around.

There's a lot more to this, but I gotta now.  Just remember, 5mm and don't touch the kitchen sink.