I was penalizing some boards and I set the copper pour wire size to zero to allow for easy placement, then I found that all the thermal wires had become zero.
So obviously that's the wrong work flow, but is there an easy way to stop the copper pour extending beyond the size of the dimension layer?
or is it just a mater of having to pulling the copper pour back from the edge of the dimension layer?
But copper pour wire size has nothing to do with placement.
Set "orphans off" then the copper pour is limited with the dimension layer, as long as there are no devices (pads) outside the dimension layer. I usually place the copper pour polygone exactely where I want the borders.
Setting the wire size in a polygon is a very bad idea, since the pour is formed by wires filling its area. With a width of zero the polygon may not even render correctly.
The dimension should always cut the polygon. It could be that the polygon has orphans enabled. This might cause the pour to appear outside the dimension box, though the dimension edges should still cut the pour according to the edge clearance parameter set in the DRC rules.
Another reason not to set the copper pour size to zero is that it produces gerbers that cause problems.
The data size for zero size copper is immense due to the number of vertices created, I can recall working on a design that took 2 days to produce the gerber for 1 layer because some zero size copper had been used.