Just as a comparison, I put a 205/55R16 in mine today. It fit in the well, but apparently the anchor point in the middle is slightly off center which is funny. Because it's so much thicker than the donut I had to flip the threaded anchor around to make it work but it is secured in there now.
For normal road use I'd probably just find a car that has the same tire diameter as whatever you're running and take the spare from it. ie the stock tire is 215/50R15 (23.5") so the spare is 135/70R16 (23.5"). 255/40R17 is 25", a Toyota Matrix uses 205/55R16 which is 24.9". I don't know what size the donut they have is but it should be ~25" which would be perfect.