Sy-uline ... How to write it dynamically

MY problem is .. whenever the linsize of my report changes , the length of sy-uline should change ..

That means instead of explicitly telling (120) sy-uline... I want to use sy-linsz .. ie

(sy-linsz) sy-uline. But its showing syntax error . I put the sy-linsz in a variable and checked .. Not okay .. somebody Please help me out.