I stumbled across this trivial but aggravating user interface glitch, and the fix. I'm posting it here in case someone else has the same problem.
Problem: Right-click on a .doc, .xls, .ppt, .docx, .xlsx or .pptx file. Choose "Open With", then select Windows Applications, Microsoft Office, Microsoft Office Word 2003. Close Word 2003. Next time you visit the KDE "Open With" dialog, the width of the dialog will be huge. It will be so wide that it might not fit on your screen.
Cause: The offender is the "Open-with settings" section of ~/.kde/share/config/konquerorrc. The command to start Word 2003 is rather long and that in the "Open With History" causes the "Open With" dialog to be stretched horizontally.
[Open-with settings]
CompletionMode=5
History="/home/richard/.cxoffice/w2k/desktopdata/cxmenu/StartMenu.C^5E3A^5Fusers^5Fcrossover^5FStart^2BMenu/Programs/Microsoft+Office/Microsoft+Office+Word+2003"
Solution: Delete the History line. Close Konqueror. Next time, the "Open With" dialog will be back to its usual window size geometry.
How I found this out: The command "find ~ -mmin -1" lists files in the home directory that have changed within the last minute. So I deleted ~/.kde and set it up again, waited for 2 minutes, then deliberately triggered the problem with the Open With dialog. The output was a short list of candidate files that might be causing the problem.