Which function to use to get the string length in Firebird?
With Firebird 1.x you need to use STRLEN user defined function (UDF). To register this UDF with your database, use the .sql scripts in UDF directory of your Firebird installation. You need to register UDFs in each database where you want to use them. If you have problems setting it up, read FAQ #130 and FAQ #203 for some tips.
With Firebird 2.x you can use the built-in functions: CHAR_LENGTH, OCTET_LENGTH, BIT_LENGTH, which return the length in characters, bytes and bits respectively.