Function ran

I am using the below code to generate random values, however everytime i execute this code in function it generates only 2 char length data, I want it to generate at least 10char data.

DECLARE
@randomstring VARCHAR(max)='',
@CharPool VARCHAR(10)= 'abcdefghijkmnopqrstuvwxyz',
SELECT @RandomString = @RandomString + Substring(@Charpool, (CONVERT(INT,Abs(Checksum(@newid)))%25)+ 1, 1)

Neil