1. I am not a big fan of ripping the OS apart. I keep mine on a SSD which gave my system a considerable boost.
2. As for your configuartion, first I would define a seperate
Data Partition. That keeps your data safe in case of a crash and you do not have to worry about the data in case of a restore. The size is a matter of the amount of data you anticipate.
3. For the OS, 50GBs is enough under normal conditions. Should you have a lot of games, 100 to 150GBs may be safer.
4. On the second disc I would allocate an imaging partition of about 250GBs. That gives you plenty of space to keep a significant amount of images. Imaging your system at frequent intervals is the best insurance against all kinds of unforseen troubles.
Imaging with free Macrium
5. Another partition I always define on the second disk is for videos. For one it speeds up image editing if it is seperate from the OS and then you have ample space for videos that can take quite a lot of room.