I wanted to resize (increase or decrease the partition size) the existing Ubuntu partition without affecting it and the other OS(es) installed in the machine.
You can resize a Windows (NTFS) partition from Ubuntu using GParted. You'll need to shrink the partition to free up some unallocated space on the drive. Then you will be able to create a new Linux partition in that space.
GParted is not installed by default, look for it in the Software Center or install it in therminal by doing
It won't be possible to modify your Ubuntu root partition (or, more precisely, any partition which is in use) while booted in Ubuntu. To do this, boot from Ubuntu Live CD and run GParted from there.
Make sure you have a backup of your important data before proceeding because it's a potentially dangerous operation.
You can find more documentation on GParted here: http://gparted.sourceforge.net/
You can re-size partitions, provided they are not in use mounted when you do so.
It will probably be easier to do this after booting up using a live disk. Not into your installed system.
As this is risky you should make sure you have a back up before you begin. I lost a whole system due to a power out-age do this. You have no excuses for not having back ups if some thing goes wrong.
You may need to install additional file systems tools such as ntfs-tools if you want to re-size partitions other then ext2/3/4.
Lots of good documentation on using Gparted can be found here. http://gparted.sourceforge.net/documentation.php
With this document giving you step for step instructions on how to re-size a partition. With nice screen shots.