See this post How Do You Adjust the Cap Height and x-height Lines?
I haven’t tried, but I guess the way to go is to create the “H” and “x” first to get the CapHeight and xHeight that you want.
What I do if I’m importing images from elsewhere is to import them all without any resizing, then resize them all together using the Glyph Transformer. That makes sure that they are all resized equally.