OK,let me make it clear!!
I am not talking about the /(root partition) here,I am talking about /root folder which is by default 2.9mb in size on dsl.
I did the usb-hdd install of dsl which by default makes two partition .One is the ext3 where all the OS data is located with the size of 50mb and the remaining are fat partition which contains mydsl extensions.The usb installation is similar to that of live CD,usb-install too contains the compressed knoppix image file and kernel boot loader etc.I guess to re-size the /root folder it should be booted from flash drive itself.
but it couldn't re-size because it was mounted.I tried to unmount but couldn't unmount it.
I guess I explain the problem as possible I can!!