I have a 750 GB HDD, what is a fair amount for the Windows 7 partition? I don't want to give it too much or too little and have to do everything over again.
Windows 7 Professional 64 bit needs about 11-15 GB for installation. It all depends on how many programs/applications you plan to put on that partition. To be safe I'd use at least 20 GB to do this. If you plan on doing this soon, put the installation on a FAT32 bootable USB drive, it will install much faster.
What do you plan to do on the partition? That should be the real deciding factor. If you are planning on gaming, I would say you should put at least 120(especially if you are using steam). If not, then I would recommend 40 or so-anything less and you will be right against the total size from the start and likely won't be able to install very much.