Your question is somewhat vague.
If you start QB in a window, you'll need to change the shortcut's properties to FULL WINDOW (like: Alt + Enter key combo).
If your Desktop is set to anything higher than 640 x 480 RESOLUTION then you'll definately have to do the above --or-- change the Display's Resolution to the aforementioned as a last resort (after you've mastered the intermediate basics of WINDOWS, this should not pose a problem).
If you are in FULL WINDOW mode -AND- QB text (itself) is teny-tiny. The reverse of what Laffalot said may be the problem. If this is the case, check your shortcut's properties for the command line, to make sure that there is no "/h" at the end.
If you are saying that a QB program is running and ITS text is teny-tiny then it is part of the program itself and you'll have to modify the code (if available). You'll have to search for SCREEN and WIDTH statements. (NOTE: if you change the source code, the program may not run at it was intended by the author and you'll end up with even more problems than you know.)
Anyways, let us know if any of this has helped or not.
--MiggyD It's better to have two heads to solve a problem from different angles than to have tunnel vision to a dead end.