Query for records which have the city length > 20
SELECT Id FROM User WHERE (NOT Username LIKE '___')
The above SOQL query will return all user rows where the username is not 3 characters.
SELECT Id FROM User WHERE Username LIKE '______'
The above SOQL query will return all user rows where the username is 5 characters.
You need to create a new formula filed of type NUMBER and insert a simple formula to it that calculates a length of the string:
LEN(Street__c)
LEN Formula
Then you can query on this field:
Select ... From ... Where FormulaField___c > 20