By OS i think you mean distribution. If you are concerned about that you have too download 3 cds, you can choose too just download the first and then get the rest over the net, or choose f.ex. redhat or debian, which have a bootfloppy that you can download, and then get the rest over the net.
If you are talking about diskspace used, when it is installed you can ghet mandrake down too 60 MB (as far as i remember), other distros can go even further.
for more info.