On a 1/8oz, you are probably not looking at anything over 3 and 3 1/2 for a double willow and that may be too much. I run 3 1/2 and 4's on 1/4 and 3/8 for double willows, with the 1/4 oz not being a burner with those blades. For a single colorado, I run a 4 on 1/4 and 4.5 on 3/8. For tandem on 1/4 I do a 2 CO/4 willow, 3/8 3 CO/4 willow. For double CO, 1/4 2CO/3.5CO and 3/8 3CO/4CO.
I know these don't translate to the 1/8, but they do provide some comparison.
For a single CO on a 1/8 head, I would go with a 3, maybe even a 2. 2 blades on an 1/8 is a lot of torque, even for willows.
Tim