I'd like to have a page that adjusts its height according to the size of the form, including adjustments when fields become visible. Is there a way to get this information and put it into a page size token?
Unfortunately, this is not possible. But from your question, I gather you are using a free-form template, isn't it ? If this is the case, event though, I am not sure that your could use this token to reformat the page anyway, at least not without some scripting.
If so, you may want to consider moving to Guided Landing Page Templates (i.e. Responsive design) that would be able to support this "naturally".
+1 to everything Greg said.
The maximum possible height of the form including visibility rules is not computed. It would be impossible for Marketo to do this on the server, in any case, since they have no idea of what other CSS you're applying to the form. (Not sure why you'd want to fit to the maximum height, anyway, as your page might like mighty strange with whitespace allocated for fields that aren't rendered.)
If you want complete control over form layout, start with something like my destyled form and then build from there.