I hope this makes clearer what i want to know...
Sorry, still not clear (to me, that is).
The size of the object cannot be the only dependent variable.
You state the background is at infinity. If that's true (for example, the stick with only clear, blue sky behind it), then the lens would be determined by the length of the stick relative to your distance from it), because the subject is already physically isolated.
But...perhaps this will help: at a basic level, OOF blur is determined by magnification and aperture (ignoring exceptions like near 1:1 macro and fisheye lenses). In your scenario, you're filling the frame with the subject, therefore magnification is not varying with focal length, although distance to subject will increase with increasing focal length to fully frame the stick. In that case, the determinant of OOF blur amount will be aperture. Thus, the 'best' current Canon lenses will be the 50/1.2 and 85/1.2.
So, that's the most OOF blur...but will it isolate the subject from the background? Depends on how far the background is from the subject. Also, a longer lens has a narrower angle of view, meaning less background in the image, and that might make it easier to find a patch of background with less detail.
Like I stated, still too many variables.
Let's back up a step - why
is this important to you? What are you trying to accomplish?