You can try the following:

Edit your mplayer config file (/home/user/.mplayer/config) and add these lines:

font = "Ubuntu"

ass-font-color = "ffff0000"

ass-font-scale = "2"

Let's say you have a video named video.avi and the subtitles in the same folder eg:(home/user/Videos) named subtitle.srt.

Then to open a video with mplayer in full screen from the command line type:

cd /home/user/Videos

mplayer video.avi -sub subtitle.srt -fs -ass

(You can change the value of "ass-font-scale" acoording to your needs.)

If you are using mplayer-gui to open the video, make sure you have SSA/ASS enable,right-click in mplayer-gui and go to Preferences.

In the tab "Subtitles & OSD" click in "SSA/ASS subtitle rendering"

If you want to change another option like font scaling just go to the "Font" tab.

(Maybe after changing some options you have to restart the mplayer-gui.)

Hope this will be helpful!

Regards.