Really you don't have to plan ahead that much. As long as you install Windows first, and have enough empty space for Linux to sit comfortably, you should be ok. As for your Linux partitioning, depending on what you want to do will decide how you want to partition it. You can have it as simple as 2 partions ( / and swap), or seperate it into more pieces. When you go through the FC install, it will mention these, and how you might want to partition it. If you are just wanting to mess around with it, the 2 partitions will do just fine.
I would say 256GB is going to be more than enough. Depending on what you all plan to do with it really determines how to partition it. If it's being used to learn Linux, and to push the learning, I would say to experiment with the multiple partitions (/, /boot, /etc, /home, /usr, /opt, and swap).