In OIM 10g we used to change the size of OOTB fields from Formmetadata.xml but in OIM 11g we don't have any such file.
- Export /file/User.xml from MDS
- Search for the field which you want to modify say "Middle Name"
- Search for
- Default size is 80, you can change it as per your requirement
- Import the file back into MDS using weblogicImportMetadata.bat/sh
- Run PurgeCache.bat/sh