I want to make B into one big storage drive, and I want A to have a small partition for the OS and a second partition for games. How much room should I leave on the main OS partition where I won't run out of space?
Yes, but the current games on that partition will not work with the new Windows install due to missing registry entries. (There are exceptions, like Steam games.) W7 has a built in Disk Manager that can do what you want.
Right click>My Computer>Manage>Disk Manager.
With W7, I'd go with nothing less that 30GB myself. I prefer larger though.
