From Second Life Wiki

The script below will give the width of the screen in meters by touching anywhere on the visible face, given that the prim is a box with zero rotation and attached at one of the corner HUD attachment points.

//Find Screen Width
//By Darwin Recreant
 
default
{
    touch_start(integer total_number)
    {
        vector scale = llGetScale();
        vector off_point = llGetLocalPos();
        vector face_pos = llDetectedTouchST(0);
        vector screen_pos = llDetectedTouchPos(0);
 
        float screen_width = ((face_pos.x-.5)*scale.y-off_point.y+screen_pos.y)*2;
 
        llOwnerSay((string)screen_width);
    }
}