Every single VSTI I own has the same problem of not being able to expand to full screen. Even the best GUI fonts seem to get smaller the longer amount of time in session I use it.
I've suggested this to every designer to add a 'full screen' option. Only one company has written me back and said they thought it good enough of an idea to try and implement (whether they do it or not remains to be seen).
All that said, SampleTank is worth the price because of it's library. (especially at the $39. w/8gb library sale price I picked it up at.) The deal now is not as cheap ($70.?) and I think you only get around 6 GB with it, but considering you get it at 50% off and another of their VIs free. It's still worth the price... GUI PITA or not.
Add: Btw, thanks for the suggestions of Windows Magnifier and/or sending it to another monitor. Going to try both as soon as I can.