The TB_SETBITMAPSIZE message sets the size of the bitmapped images to be added to a toolbar.
TB_SETBITMAPSIZE wParam = 0; lParam = (LPARAM) MAKELONG(dxBitmap, dyBitmap)
Returns TRUE if successful or FALSE otherwise.
The size can be set only before adding any bitmaps to the toolbar. If an application does not explicitly set the bitmap size, the size defaults to 16 by 15 pixels.