What size are my trailer rollers?

[sigsmaster]

I have a calkins trailer and i need to replace the keel rollers, but when i measure with a tape measure it looks like they are 9", but they don't make 9" rollers. all i can find are 8" and 10". 8" would be to small and i'm afraid that the 10" won't either. can anyone help?

 

[Lazy Daze]

Measure between the mounts. Buy a roller based on that measurement.