Are you going to do a clean Windows installation? If so, Windows setup can handle that for you. If you are adding it as a second drive for data storage, once installed go into Windows Disk Management Utility. You can create the partition(s) and format them by using the right-click menu on the graphical representation of the unpartitioned drive.
i normally go with 50/50. you want to make sure that you have plenty of space for installs on your c drive for apps/games/utilites ... etc.
but you also need to make sure that you have plenty of space for pagefile swapping.
if you run out of space on your c drive, your entire system slows down.