You can look at the image properties to estimate more precisely.
pos.gif is the green part
neg.gif is the gray part
cleardot.gif is used for some sites as well
The total width equals 40px.
In decreasing PR, the sizes used are:
Some food for discussion:
There are only 8 notches here, and even a PR10 site like Google/Yahoo uses only the 38px image.
I also looked up a "cleardot" listed site, and the corresponding Toolbar PageRank shows as 2.
I'm not sure how they scale Directory PageRank compared to Toolbar PageRank, or if the two are even related.