The dialog box itself is not resizable.

If you're finding the same term repeatedly, after you use the dialog once
you can click the Cancel button and then use the double arrows at the bottom
of the vertical scroll bar (or the shortcuts Ctrl+PgUp and Ctrl+PgDn) to
jump to the next location of the same term. However, this works only for
Find, not for Replace.

The only other alternative I can think of would be to intercept the Find and
Replace commands with macros that would run custom userforms
(http://www.word.mvps.org/FAQs/Userforms/CreateAUserForm.htm and
http://www.word.mvps.org/FAQs/MacrosVBA/InterceptSavePrint.htm) that are
smaller. They might omit some of the options and buttons of the original, or
pack them closer together.