I'm doing an amplifier with 50 Ohm input impedance, so that is going to have some non-trivial currents running. I can just make my wires and transistors... big... but not having a ton of real taped out experience I have no idea about what's reasonable. I can imagine there are not any advanced power analysis tools at this point, but... are there any constraints from the vendor or rules of thumb that you can manually work out how likely it is that your design will not go up in a puff of magic smoke?

As a rule of thumb, for aluminium of usual thicknesses, if you stay below 1mA per micron of track width you should probably be OK over the commercial temperature range. It gets a lot worse as temperature rises, so in the even that it is marginal you could just keep the chip cool to prolong its life.

Thanks, that's very useful to know. What about the transistors themselves?

I don't know. At least you can make sure the metal fingers on the source/drain are below 1mA/um, but I have no idea whether there are other parts that might fail. Different materials have different susceptibilities to electromigration.