Updated documentation so it's clear you should use SDL_SetWindowDisplayMode() to change the size of fullscreen windows.
diff --git a/include/SDL_video.h b/include/SDL_video.h
index 52d6ead..6f6cda8 100644
--- a/include/SDL_video.h
+++ b/include/SDL_video.h
@@ -604,8 +604,8 @@ extern DECLSPEC void SDLCALL SDL_GetWindowPosition(SDL_Window * window,
* \param w The width of the window, in screen coordinates. Must be >0.
* \param h The height of the window, in screen coordinates. Must be >0.
*
- * \note You can't change the size of a fullscreen window, it automatically
- * matches the size of the display mode.
+ * \note Fullscreen windows automatically match the size of the display mode,
+ * and you should use SDL_SetWindowDisplayMode() to change their size.
*
* The window size in screen coordinates may differ from the size in pixels, if
* the window was created with SDL_WINDOW_ALLOW_HIGHDPI on a platform with
@@ -613,6 +613,7 @@ extern DECLSPEC void SDLCALL SDL_GetWindowPosition(SDL_Window * window,
* SDL_GetRendererOutputSize() to get the real client area size in pixels.
*
* \sa SDL_GetWindowSize()
+ * \sa SDL_SetWindowDisplayMode()
*/
extern DECLSPEC void SDLCALL SDL_SetWindowSize(SDL_Window * window, int w,
int h);