Nope, still can't confirm with multibyte editbox (And none of the other 4 options) enabled. I made the font large, and once again tried with Tahom and Fixedsys, and tried 5+ times. The font size just doesn't change in the editbox.
Start > Control Panel > Regional and Language Options
Do you have a different language set that might use non-alphanumeric letters? (i.e. circumflexes etc.)
As you've both experienced this, and I've followed your instructions to the letter thus far and I am not experiencing it, I guess it's got something to do unrelated to mIRC, most likely Windows settings. Multibyte editbox does seem like the likely culprit however, as the problem is occuring in the editbox.
Regards,
