This is a difficult one. I think the solution lies not with Inkscape but with your GTK theme. The best lead I can give is to point you to the gtkrc files which may be found somewhere on your system. (I'd give more specifics but I am not a Mac user.)
GTK? Sorry, you've lost me there.
Notice how Inkscape is kind of Grey and funny looking (not like a normal Mac app)? That's GTK. ;-) GTK looks great on Linux, not as great on Mac. GTK determines the font size, among other things, but no one so far has known which files to edit to change the GTK settings.
I think GTK is roughtly the same things as "Aqua".
Essentially, it's not exactly an Inkscape issue, it's a GTK issue. Inkscape *does* use GTK, but so do thousands of other programs. -- Michael Moore ------------------------- Creative photography for any occasion http://elementsbycaroline.com Full quality proofs Online album