problem assigning length to a parameter on selection-screen

Dear Experts,

I need a text-field on selection-screen that can hold 160 characters. For that I have declared a parameter of length 160 (string). But the requirement is that the total size of the block must be visible on the screen itself. Now when I'm typing the letters

get shifted to the left as the visible length of the block exceeds, but the whole text should be visible at a glance.

Please help me.