You can download a netiso (net install / minimal iso). That kind of ISOs are between 80-250 MB (or even less) and contains a base system only but you can download the "rest" of the system from internet.<p><a href="https://help.ubuntu.com/community/Installation/MinimalCD" rel="nofollow">https://help.ubuntu.com/community/Installation/MinimalCD</a>
<a href="http://www.debian.org/CD/netinst/" rel="nofollow">http://www.debian.org/CD/netinst/</a><p>But If you are not confident enough with Linux perhaps you might consider other options.
Porteus runs in under 300mb and runs nicely off a cheap USB stick or CDR while having all the basics including a Desktop GUI.<p><a href="http://www.porteus.org/" rel="nofollow">http://www.porteus.org/</a>