There's a freeware utility called BR4, with which you can reindex dbase files. BR4 also has the ability to create new dBase files, and alter the structure of existing files.
With BR4, you could change the size of the field you mentioned. But be aware that if your application is designed to do any kind of maintenance to the dBase file, it may rebuild the file and set the field back to it's original size, based on a data dictionary stored in the app.
Do a web search on "BR4" - you should be able to find scattered through the web -- or go here- ftp://ftp.goldmine.com/Public/Utility/ and look for BR.ZIP
One caveat on BR4 -- it's a DOS utility, and will choke on very large files (not sure of the exact limit - somewhere in the 200MB or 200k record range). Make a backup of your files before attempting to change anything (duh!). If your file is larger than that, there's another utility called BR7 that's Windows based and doesn't have the same limitations.