I've never seen a TEXT field show "?" when characters are exceeding the field display length. Could it be a NUMBER field instead?
And can you clarify "truncate"? Field Definitions can have "Validation" to limit the length (# characters), but that would display a dialog to the user. Do you just want the field to disallow entry of # of character beyond?
What does font size affect the field? For example, if the font is larger, less characters may be displayed.
"I've never seen a TEXT field show "?""—neither have I. Usual thing with text fields is you just see what will fit, but the full text can easily be revealed by clicking into the field—or you can set a scroll bar.