Well, we did consider that Jenny would [i]try[/i] to make herself smaller for practical social reasons; using mad-science.

It could be that your girl would use a related approach (to make herself bigger) that could later be used as a story-arc.

Common sense is telling me to refrain from using anything called "Pym Particles"
-Probably going to use similar limitations, since no point re-working the biophysics

So likely a max size of 100 feet before cardiac failure simply standing still, 25 to 50 feet being more typical.
-Inverse square cube law