Bytingshark - user.js who will try show to you currently playing songs file size. Works on all modern browsers like Firefox, Google Chrome, Opera and (maybe) Safari.
P.S. I haven't tested newest version (2012.10.23) in all modern browsers. And some act strange for no apparent reason (Opera). May come back to it, some other day.
You will need to install Greasemonkey add-on. After that just open bytingshark.user.js and install it.
Open Tools > Extensions and enable 'Developer mode'. Then use 'Load unpacked extension...'.