If I'm mostly going to be on roads 25mm tires work perfectly well and can handle the limited amount of rough stuff. Where they have real trouble in is mud and sand.

If you are going to be on rough stuff a lot, through a winter, you'll need wider tires with more tread....check out Schwalbe Marathons Duremes etc. They worked well for me
on a mix of rough roads and gravel. Here is my bike with 35x700c Duremes