An application sends a ZGM_SETROWNUMBERSWIDTH message to set the width of the row number column. This column cannot be adjusted by the user, so it is the only way to adjust this column's width.
ZGM_SETROWNUMBERSWIDTH
wParam = (WPARAM)(int) iWidth; //Width in pixels
lParam = 0; //not used; must be zero
This message does not return a value.