Footprint smaller than inner or outer tent?

Yes, smaller than the inner’s footprint.

Thank you. Not really sure why, though?

Because if it just smaller than the fly it may still collect rain from splashes or wind blown rain.

It then gets sandwiched between the waterproof groundsheet and the tent floor, so that  it can penetrate the floor with the full body weight on top.

How much smaller tho?

Is there a guide or general rule or do we just play it by ear?

Given that both Tyvek and PolyCryo shrink in the heat (when washed for the former) and most others accumulate wrinkles anyway, I would start with the exact size of the floor. In other words I would opt for something that with use becomes just a little smaller than the floor.

I have always used polycryo or more recently tyvek groundsheets cut to the size of the inner tent.  Over time do to wrinkles and shrinkage they probably end up being slightly smaller than the inner tent, but since it is just an inch or two around the edge and most of the time you aren’t putting much weight right at the edge of your inner tent its not an issue.  You could probably make it larger than your inner tent as long as it was still far enough away from the edge of the fly that it would not get rain on it, but I don’t see any advantage to doing this unless you wanted to have a floor on your vestibule, but even then I think you would probably be better off just using a second piece of tyvek cut to shape for that.

And yes a polycryo ground sheet cut just smaller than the inner tent is really, really worth it. Also, clear polycryo allows you to see any prickles that might puncture your pad or make you uncomfortable.

I haven’t tried tyvek.