Don't open an avchd file in thumb nail

That little square is the default size of Windows Media Player in its 'Now Playing' mode.

You can set WMP to resize itself automatically to the video once it has opened it and determined its size.

Open WMP, either by opening a video or preferably by running WMP from Start. If you opened it by playing a video, switch to its Library view (little icon, top right). Running it from the Start menu will open in Library view.

On the 'View' menu, under 'Video size', tick 'Fit Player to video' and the player will snap to the video size as it plays it. You may also like to tick 'Fit video to Player on resize' which will scale the video if you resize the player window while it's playing.

You can set WMP to open initially to any size you want, you're not stuck with that 'postage stamp'. Open WMP from the Start menu (click Start, then type windows media player to find it). Click the little icon bottom right to switch to 'Now Playing'. Click and drag the window to the size (and position) you want. Close WMP and it will remember these settings and use them next time it is opened.

Ken Lichtsinn said:

I don't see a view menu...

Ctrl+M will toggle the menu bar on or off.

Alternatively, Alt+V will bring up the View menu without the need for a menu bar.