Hi, I'm using ubuntu 10.10.While I installed it in my desktop, i choose double boot by selecting "install them side by side,choosing between them each start up". After installing I found that my ext4 with root size is 100GB and swap 4 GB. Isn't it too big!!?? Is there any way to reduce the size of ext4 with root without harming the OS? Should it be done?

You'll have to boot onto a live CD, and unmount that root partition. Its probably set up as a volume group. Here are the pseudo steps to do this:

Make a backup of your computer (clonezilla is nice for this)

• Boot to a live CD
• Unmount the root drive (if it is mounted)
• Resize the filesystem with resize2fs
• Reduce the size of the Logical Volume with lvreduce
• Reduce the size of the Volume Group with vgreduce

The / partition has all your OS on it unless you installed on 2 partitions (/ and /home). Look at the size of your other OS. I really do not think that 100GB is too big.

Debian is my main OS. This install is about 6 months old. I am installed on / and /home (system files in / and data in /home. My root partition is 30Gb and half full already (I use a number of programs most folks go with 10 or 15Gb). My home partition is only 51Gb but I keep a lot of files on another partition that is 100GB and has over 61Gb of files in it.

As you are on 100GB (one partition) I added all mine together and come up with 96.75Gb used (Gparted numbers).

I would be extremely surprised if Ubuntu installed with LVM as a default install.

Easiest would be to get a gparted liveCD and do it all from there. This is a GUI similar to Partition Magic say - just grab the edge of the partition and move it appropriately. Hit the "Apply" button, and it's all done for you.

Indeed. In order to install Ubuntu with LVM, you have to install Ubuntu from the alternate install CD, and select to install using LVM. Using LVM has never been the default in Ubuntu.
By default, the Ubuntu installer will create two partitions. Your swap partition will be roughly 2x your memory. The root partition will be whatever is left over.
In order to install Ubuntu with a separate root, swap, and home partitions you have to select manual partitioning and choose the appropriate sizes of the partitions, and mount points for each partition.

thanks to all....:-)