Ok so you wont be doing server stuff and development so you wont need as much space.
I would say the actual OS and stuff will probably use between 4-7 Gb but thats an estimate and it can vary a lot depending on what packages you install.
I would probably make the partition at least 10Gb probably between 15 and 20 gig to be safe. Whats your Hard drive size and how much space do you have left.
If you have a big hard drive thats over 200gig then try making a 20-40Gb partition for linux and files and stuff because once you have installed it, it can be very hard to repartition. I once did it and corrupted the partition losing my whole website, all files in linux and had to renstall the OS.
So allow more than you need. probably 30Gb on a big HD.