I just wanted to make a very simple animation and then this... Why does it change its size!?

I have Vista x64, it's the same on XP x32.

[video:youtube]http://www.youtube.com/watch?v=T-ceSN_DoiQ[/video]

... and why does the video function not work?

This is a frequent mistake with animation, look here,

http://manual.3dgamestudio.net/med_faq.htm

You had not enough frames. Your model shrinks because vertices move along straight lines between frames. If you had only two frames it would even shrink to a single point.

BTW why dont you rotate it with a script?

Ah thank you, yes now it works, so its not a bug.

And I don't rotate with script because the wheel is one of many enemies (yes it is) which have all the same action so it's the easiest way to make a walk animation for all of them.