How was this BOX created, using Insert->Box, or by setting up Borders on the field itself? The latter will just expand.
If you have the Can Grow on, you can also select the maximum number of lines alongside of that so that it will not expand out of a BOX. Or you can trunctae the field by using something like:
left({table.field,X)
Where X is the max length you want the field to go before truncating the rest of the text.
You can also dynamically shrink the size of the font to allow more characters.
-k