Roughly, 31.9 is my lowest and I want lower. I have an 11-28 cassette, compact double 50/34 on the front, and 165 cranks.
The issue with going to a cassette on the rear that has more teeth than your 25 is whether your deraileur can handle it. I have a short cage derailleur on my bike and it isn't rated to handle a cassette with more teeth than what I have.