Reading these pages years ago was a bit of a revelation for me. I had never heard anyone talk about this topic before, and I still have never seen anything that has written about this or seen it mentioned in any form.
I will do my best to try and paraphrase what (I think) Vance meant by that and make it as clear as I can.
Basically, here's my interpretation: Vance was using the term "scale" to say that, when you design and draw objects, you should emphasize the most distinctive parts of the object to make them recognizable to the viewer and make them read clearly. He also used "scale" to mean that you should de-emphasize the uninteresting parts of an object to make the best drawing you can by not wasting space on uninteresting parts. And lastly, and most importantly, he talked about "scale" as a way to make things (and people) seem closer to the viewer, more intimate and more expressive.
Here is the entirety of his thoughts on the matter:
Vance used Robert Crumb as an example of an artist that uses "scale" well.
This one he captioned "R. Crumb...ugly but have great scale". Hopefully that won't offend any Crumb fans out there.
I have always found these pages--a rare glimpse into the way Vance's mind worked--fascinating.
Scale is not an absolute value, at all. Obviously, it is very subjective and depends on the subject matter. For example, big balloon-y cartoon tires work just fine if you're drawing a car in a Mickey Mouse short, but if you're storyboarding a movie like "101 Dalmatians", you can't draw Cruella's car with big balloon tires. It would undermine the menace of her character and look completely inappropriate for the art direction and tone of the movie. So you have to find a way to draw her car (and tires) in a way that's simplified and reads without being "out of tone" with the subject matter.
Many great artists and illustrators have developed their own sense of "scale" to emphasize certain parts of their figures and environments to get the results they desire. Take a look at some of your favorite artists and see how they have adapted this concept, and what results they have achieved by interpreting reality with a unique sense of "scale".
If anyone would like more clarification (or has some clarification to add to my interpretation), please let me know!
If you're interested in seeing more of Vance's work, visit Ed Gombert's "Vance Gerry Memorial Blog."