The SetWindowLong function changes an attribute of the specified window. The function also sets the 32-bit (long) value at the specified offset into the extra window memory.
这个API似乎与修改窗体大小没有关系。
你可以使用SetWindowsPos修改窗体大小。
The SetWindowPos function changes the size, position, and Z order of a child, pop-up, or top-level window. Child, pop-up, and top-level windows are ordered according to their appearance on the screen. The topmost window receives the highest rank and is the first window in the Z order.