Previously BT5r1 auto-size guest display worked perfectly, however I did a distro upgrade from r1 to r2 and got problem doing auto resizing.
Well here’s how to do it:
1. prepare-kernel-sources
this script is included with BT, great script for downloading linux kernel source codes. Executing this script will remove previous source code (linux 2.6.39-4) and download the current linux version source code.
2. Install guest addition with virtualbox.
3. Reboot and you are done.
cyruslab 
Thanks a lot! I wouldn’t have thought it’s so simple. The 3rd step for me was to end an X session and start it back again.
You are welcome. I am glad that this post helps 🙂
Thanks a lot for the hint of re-installing the guest additions – it helped me to resolve my guest system display re-sizing problem 😉
I’m glad the post helps 🙂