I use rollers for all my riding in the winter, and I've found something like a 52 x 17, using a 700C x 25 tyre to be about right. That combination gives me a GI of around 80 which is comfortable; 20+ mph at a cadence of ~90; fast enough to be able to stay balanced on the rollers, and be able to practice "single leg" exercises or ride "no hands".
- Wil