j_bins - I'm sure someone will comment with better info than me, but as far as partition size goes for OS - my recovery drive is 23GB so I'm guessing that's the size of the OS.
It is possible to shrink the existing partition then use the new unallocated space to create/format a new partition without loosing data, programs and the OS? And this can be done with no risk?
Well I doubt there's ever anything that's objectively 'no risk', but as far as I can tell it's a pretty common practice, so I wouldn't imagine that would be any drama around it...
How many partitions are you going to have?
1 OS / Programs
There are probably other combinations as well... I'm not sure what's best / most secure / quickest. I imagine that having them more separated might make any future changes easier to implement, I don't really know though.
I would have thought that the partitioning would have been done after the install, and then assign documents / music / downloads etc to the new partition.
Like I say, I haven't actually done this, Interested to hear what others say though as it's on topic.
FYI my program files / program 86 files folders combined size is 25 GB. I don't have games installed, I have a fair few programs kicking about though. So perhaps 100 GB would be a conservative number for the programs, seeing as you have a fair size drive to play with.