You can install along ANY other operation systems. The size of hard drive mandrake needs, depend entirely on what you install. 3-4 Gb is a good idea, as it will leave you with enough space to install aditionaly programs, create movies, and such. But if hard disk is scare, you can proberly get away with 1 GB including user files. Be warened though, that you then will have only 2-3 hundred megabytes free
I will look forward to welcome you to the free world.