from the tree view-- System Navigator>System Definition>Tables
you can see the tables then expand the table name you want to modify and you will see fields.
click on the field you want to modify and it will display all information to need. The fields in the ServiceCenter (p4) tables are dynamic expanded to fit what data saved, but you may need to open the format to allow more data. The tables have a default 2 gig limit.
for DB2, you need to modify the DB2 field size first using a database tool.
Then follow the same steps as before but at the bottom there is a tab called SQL tables. click the link from that screen called open SQL utilities menu you should see the ServiceCenter field and the mapping for the DB2 table. Change the field mapping to the correct size and click continue. this will modify the mapping to match the DB2 table. Note, sometimes you will need to bounce the ServiceCenter server to have this pick up the changed size.