The character limits that USPS places on the address fields on their labels are:
Name - 38 characters
Street 1 - 36 characters
Street 2 - 36 characters
Street 3 - 36 characters (for international -- API only)
City - 15 characters
State - 2 characters
Note: USPS will not return an error if you exceed these limits, but will cut off the field at that limit.