If you have ever watched a wide-screen foreign film with subtitles you might have noticed that the subtitles are usually put inside the picture. I find this extremely annoying as it makes the subtitles harder to read. It doesn't make much sense, if you already have black bars from the aspect ratio adjustment why not use them for subtitles? Fortunately if you use mplayer you can. Just add the following to your personal mplayer config file ~/.mplayer/config or the global /etc/mplayer/mplayer.conf

ass=1
ass-font-scale=1.5
ass-use-margins=1
vf=expand=:::::8/5

You need to adjust the last line to the aspect ratio of your screen. As a side effect all videos (even in windowed mode) will have the black bars added to them to pad them out to the aspect ratio, its a small price to pay.