The size of the password for Self Service is limited to 12. If you try to increase the max entry size, you will not be able to increase it beyond 12 through CRM. However you can do this by an out of box concept given below.
The password field comes from the Visitor table from CRMSelfService database. If you increase the Entry Size from CRM but it did not work, you can easily increase the size of the password filed for from backend. Copy and paste the below script in the Sql Server Query Analyzer and execute the Script using F5 key.
USE CRMSELFSERVICE
ALTER TABLE VISITOR ALTER COLUMN Visi_Password NVARCHAR(60)
USE
UPDATE Custom_Edits SET ColP_DataSize=60 WHERE ColP_Entity=’Visitor’and ColP_ColName=’visi_password’ and ColP_Deleted is NULL