The old thread in the enlarging forum concludes the correct formula for exposure compensation when changing print size relates to the change in the magnification. The amount of exposure change required depends on the size of the negative.
The formula is ((M + 1)/(m + 1)) ^ 2, where M is the new magnification and m is the old.
When computing this for various negative and print sizes:
It seems the old "one stop per paper size" adage isn't very accurate. Although 0.5 stop may not seem like that great an error, it is an entire print Zone in tone. Certainly nobody will argue that a 14 second print looks quite different from a 10 second print ...