what is the minimum space of pen-drive required to install ubuntu in a pen-drive.
ubuntu may be of earlier version

Probably around 1.5 gb I would imagine

2-4gb

You can save a lot of space if you only need command line.

Or just install a smaller distro

can i get more information about installing smaller distro
or what can i do with only command line. could i be able to boot any new computer.

you need a small and GUI , well use the DSL ( damm small linux)
. Even you don't need a good graphic card to run this ( very light window manager , I just using it with a old 486 laptop computer
waste to our country ).use the xubunthu it's also light weight.

anyway if you feels it's very small and you need a small distro
with GCC and other developing tools, just use knoppix.

I just downloaded that knoppix CD and run these scripts and make
it to a bootable flash drive.
http://www.pendrivelinux.com/usb-knoppix-510/

what is your purpose ? what kind of tools or environment you need.
( above I said about a minimal environment to run the linux GUI system and C/C++ compiler and debugging toosl. best for learning
QT ).

my purpose is to setup linux in as small as possible space in my memory stick and boot my old p4 computer with 128mb RAM and xp installed. i dont want to remove xp. and as i have low disk space i dont want to install 2 operating systems

Check out tinycore! (http://www.tinycorelinux.com/)
Or P.U.D. Linux! It's based on Ubuntu.
SLAX also runs great from a pen drive!

For the record, I've run the standard xubuntu on a 2GB stick with no problem, but it was a little cramped for space. I fixed that by setting it up to auto-mount the home directory on my web server if I'm connected to the internet at boot-time.

Like Nic mentioned though, it really depends on what you intend to do with it, what kind of tools you need, how much room you need to store stuff, etc...

-G

If you don't want to install a second OS, then why not just use a CD/DVD distro? Something like ubuntu/knoppix or even some of the more recent Fedora Spins?

RAM may be an issue, but you can probably use a swap file on the windows partition (like windows does itself) which is pretty safe to do.