What size of jar, proportionally, should I keep my poolish in to prevent overflow?
This is after I saw it hitting the top last night and stirred it down before bed.


You have been warned. But it's certainly not a bizzare fluke.

What could have helped the dough out:


at 100% hydration and low volumes you have a good chance that you get less-than-90% hydration dough by only a slight mistake in measurements; 90% dough can actually be pretty stable to support itself all the way up (especially if you have some better bread flour)
there might be a giant heap of nutrients left for the yeast/bacteria after 2 less-active fridge rests
your jar is quite narrow, which in fact provides lots of support to rising dough


In my opinion, to your original question, filling 1/3 for counter rests is on the edge, 1/4 should be okay. You can add some extra safety by having a wider jar, wetter dough, and feeding the starter much less for fridge rests (I actually always let the culture feed on the new flour for a day or two on the counter, before putting it to fridge).

About your "why"s:


Just 1 gram of consumed sugar produces ~0.248 litres of carbon oxide. Not bizzare at all, just a bit of bad luck that your dough was strong enough to be able to trap it. :]
IMO yes.
Biga is a short-lived starter culture, people usually don't keep it for longer than 1 day. Given there's much less water (40%) the dough usually starts to tear instead of rising very much. Btw. poolish is usually also taken to be short-lived; isn't your poolish actually sourdough-ish?