thanks - I have more insight into problem after playing around.
Resetting the size of an equation that is embedded in a paragraph of
text seems to do the right thing, i.e., 10 pt equation size = 10 pt
text size in paragraph. Yet resetting the size of a free-standing
equation does not seem to match. It seems to me that 12 pt equation
size (free standing) is same as 10 pt paragraph text size.

Furthermore, the sizing algorithm doesn't appear to be consistent.

Suppose there is a Full symbol with an overscore. That seems to get
"fitted" to the pt size requested, e.g., 10 or 12 pts.

But the symbol below the overscore is thus shrunk, as are all other
symbols without overscores in the same equation.

So the result is that the equation size looks too small.

Evan