On the ps4 about 110gb is taken up by os and online and settings. Average game install is between 45-80gb plus game saves if on console but can use the cloud to save these. New games like battlefront and need for speed are all saved online now on a game profile but this makes the install slightly bigger.
Ive put a 3tb in mine out of a old laptop i had lying about and im nearing full on that but i do have a good few games mostly digi downloads but you can delete these if needs be and re download again via the store or library for free as you have already paid for them.
Xbox one supports game install data via any external usb 3.0 hdd sony does not.. It only accepts media from a hdd as sony belive running games from a external hdd is 2x slower than if from internal memory i do notice some load issues and slight lag when i play games from my external hdd on xbox one tbh but sony are ment to be releasing a update to allow the use of external drives.