What is the standerd size of partition in windows 7 ?

I recommend either a small separate drive, or a partition 60 Min -> 100 gigs. You not only need space for the operating system and for programs, you also do not want to get below 16% of free space. i.e. if everything would fill 50 Gigs, then the Partition should be no less than 60 gigs. Defragment requires some free space to shift files around and you have to allocate for hidden files. The stated requirements are 16GB for 32 bit, and 20GB for 64. Just keep in mind that the minimums are for the OS only. If you are planning to install programs onto the system on the OS drive too, you'll need more space. The more and the larger the applications, the more space you'll need.