What do you mean by scale? Do you mean the number of degrees in each direction covered by the maps?
I just took a look at Chart 5, then looked up the angular separation between Denebola and Menkent (60d 24m 42s), which are aligned pretty much diagonally across that chart. Increase that by about 8% and you've got a good estimate of the diagonal scale in terms of degrees of sky coverage.
I then checked Chart 15, looking up the separation between Menkalinan and Alnitak (47d 04m 33s), which are aligned along the long edge of the chart and almost fill it. Those measures seem to show the scale is roughly the same (within a few percent) for those two different charts.
I don't know whether the relationship holds for all of them. But if it does, you could measure those two in mm and use the figures above to get mm/deg if you like.
Notice the charts are different sizes and shapes, but the scale may be the same. Then again ...
However, I don't think those charts are intended to be astrometric (accurate measures) ... they're just a matter of convenience for those who are starting to learn the sky. If what you want to do is measure separations, use a program like The Sky or a good atlas like Sky Atlas 2000, which comes with an overlay for measuring.