You need to create your own module which inherits, then changes the python class definition for the account.journal model.

This example changes the size from 5 (what is in in the core codebase) to 9.

class account_journal(osv.osv):
    _inherit = "account.journal"

    _columns = {
        'code': fields.char('Code', size=9, required=True, help="The code will be displayed on reports."),
    }