I don't think the devs have any control over that. IIRC the only customisation you can do of the payment pages at ProtX is to send them a 200px by 100px logo for them to include.
You should send that screenie to [email protected] though. Assuming you haven't accidentally set IE's default text size too small. A lot of our customers use ProtX so I'd rather you got them to fix it now before I get the barrage of support emails from them tomorrow.
EDIT: argh, turns out it was the IE setting. I had previously ruled this out because it didn't change when I pressed ctrl and +. But apparently that hotkey is Firefox only.