It seems that the size of preview window are fixed, though the window for WORD documents look bigger. And they are always horizontal square boxes that seem to make a consistence with the screen appearance. Is there any principles to design the shape? What would happen if make a vertical window's shape?
The preview window on my mac is definitely not always the same size (I'm assuming you are talking about the window that pops up when you click on a document and hit the space bar).
For example, if I preview a video, it's wide (just like the video), but if I preview a pdf document, it's tall (just like the document itself).
If the document is rather small, it displays its real size & dimensions.
I do notice that the window is never more than ~1460px wide and ~740px tall on my monitor set to 1600 x 900 resolution.