Assuming you're speaking of the dropdown available from the Forms
toolbar, there is a hard-coded limit of 50 characters per entry (and
also a limit of 25 entries). There is no way to change that.
If you use the ComboBox from the Control Toolbox toolbar, there's no
hard character limit. However, if an entry is longer than will fit on
one line at the width of the box, the rest of it won't be shown.
Setting the display font of a combobox to 8 pt Arial Narrow and making
the box 6 inches wide, I could just fit the entire first sentence of
the Gettysburg Address (176 characters). The combobox control that you
can use on a userform has similar behavior.
Another alternative that might work for you is an AutoTextList field
(see
http://word.mvps.org/FAQs/TblsFldsFms/AutoTextList.htm). The
items in the dropdown (the names of AutoText entries) would have to be
short descriptions, but choosing one would result in display of the
entire entry -- which could be as much text as you want.