By page width, do you mean the width of each tab in the tab strip above the page area?
If so, I guess you are referring to the pixel setting in Lunascape Settings - Tab - General? If this is correct, could you tell me the exact sequence of motions you perform to change the setting? E.g., a) check the box Fix width for tab-title bar, b) scroll up to 160 in the pixel box, c) click OK
If I misunderstood you, could you tell me exactly where (which menu, which dialog) you are trying to change the setting?