I have a problem with the guitar neck and fret board moving up when I change chords. I try not to put to much weight from my right arm on the body of the guitar but that weight causes the guitar to rotate when I chord change. 

I have a 41 inch length guitar which I read is the usual for beginners. It does feel a little big but I think it's the right size for me to get used to. 

advice please on how to keep the fret board still. 

thanks