The TCM_SETITEMSIZE message sets the width and height of tabs in a fixed-width or owner-drawn tab control. You can send this message explicitly or by using the TabCtrl_SetItemSize macro.
TCM_SETITEMSIZE wParam = 0; // not used; must be zero lParam = MAKELPARAM(cx, cy); // new width and height, in pixels
Returns the old width and height. The width is in the low-order word of the return value, and the height is in the high-order word.