I don't think that you can change the length in the click and point interface. But you can surely do so in the generated code : just look for the SQL procedure, then in the select statement, you have
substr(nrrelat,1,9) AS computedVarName FORMAT=...
Add between the new column name and the FORMAT option the following code :
LENGTH=9
and re-run the code.
That was not really what you were looking for, I think, but I can't think of any non-coding way to do that.
Regards
Olivier