1- Is there any other way to watch the videos full screen?
-fs. Bus is probably not fast enough to transfer 640x480. For 320x240 it is sufficient, so you have to scale video down and use -vm. It works ony for -vo pxa.
2- Do i have to change to Angstrom X11?
No, you van use -vo sdl, -vo fbdev or -vo pxa.
3- Can i just install x11 libs and use "-vo x11"?
No. You also need an X server, then run the server, then set properly DISPLAY (and XAUTHORITY; or start X with -ac), wait for X being started. Then you may run mplayer.