The following command tells the available commands for keyboard input.
mplayer -input cmdlist
The keyboard shortcuts to mplayer are stored in two locations - /etc/mplayer/input.conf and ~/.mplayer
I prefer editing the /etc/mplayer/input.conf.
Assign shortcuts to aspect aspect ratio switching as follows
a switch_ratio 1.3333 # 4:3
s switch_ratio 0 # switches back to original aspect ratio
b switch_ratio 1.7777 # 16:9 ratio
put his at the end of /etc/mplayer/input.conf.