I replaced all of the parts on an old Vintage 6-screw trem but the hole for the
trem bar is off by about 2mm to one side and its in the way when I screw in the bar.
What can I do to make it bigger?
If I read that right, your trem arm is too close to one side, correct?

If so, why make it bigger? You'd need to move the hole (which is impossible unless you know of something that can fill the current hole).

But if you actually do just need to make it bigger, just drill it to the desired diameter and re-tap it. A tap is a tool (actually it's a bit) that is used to cut threads in holes. Hope you have a tap-and-dye set...
If I understand right, the trem is hanging over too far right and over the side of the original route?
I'd just get a dremel with a rough sanding attachment and sand it down, if it's that close. it's under the trem, nobody will ever see it.