Under Windows, open_graph dimensions do not take into account the height of the title bar, meaning it takes some default 17px from the top of the window things (I don't know exactly what), which means that the window is cropped at the top and at the right, which has to be manually compensated.
Also, from OCaml we cannot see the actual height, which is customizable, but the C code could.

On another note, resize_window is not implemented, and it certainly would be great to have it (else as of now the window always has the size defined in the open_graph).