In Pd Vanilla and possibly Pd-l2ork, tk menus and widgets should "do the right thing" because of something called tk scaling:
http://wiki.tcl.tk/8484

In Pd-extended, "tk scaling" is hard-coded to "1" in pd-gui.tcl, but if you remove that line things should scale up accordingly.  (Or reveal bugs in tk if things look wacky as a result.)

You won't be able to scale iemgui sizes in any flavor of Pd.  They are specified in pixel units.  There is a canvas "scale" subcommand but the catch is that text items (i.e., most of the content of a patch) doesn't actually get scaled.  Worse, tk canvas seems only to support integer font sizes, making it burdensome to try to use canvas scaling and manually tweak the font sizes to match.  (I think Pd-l2ork currently does the reverse-- you choose a different font size and it picks a corresponding scaling factor, though I don't know if it actually uses the "scale" subcommand or not.)

So burdensome is scaling text on a tk canvas that someone just decided to make their own mono-spaced vector font in tcl/tk using canvas curves, demo'd here:

http://www.youtube.com/watch?v=n3sz_KRdRmY

In Pd-l2ork, canvas zooming is essentially one very tricky step away.  It has a scalable canvas text item called "ptext", but unlike the normal text item it doesn't have editing features (i.e., showing the insertion caret, highlighting text, etc.).  So someone would have to code that up manually.  But once that is done you could scale the entire canvas, or even rotate/skew it if you're into that kind of thing. :)

Btw-- the potential use case you mention shows the weakness of using images in patches.  For example, if you made a button that toggled between two png images when clicked, that button would look blurry when you scale the patch up.

-Jonathan