There is a Microsoft Connect issue on this, and the issue was closed, saying:
The Find Dialog growing is a known
issue that we will be fixing for SP1.
Thanks,
Weston Hutchins
Program Manager - VS Platform
http://blogs.msdn.com/visualstudio
There's actually a couple other duplicate cases, but no posted workarounds.
I don't know how long it will be until SP1 comes out. The answer in this post suggests 6-11 months after release, which would put it at October 2010, at the earliest.
The "workaround" that I'm currently using is to simply dock the Find and Replace window in the IDE (I personally put it on the left side under the toolbox). Obviously, this doesn't allow the window to widen every time you use it.