With having two of the same OS's on my computer how can I pick which one to boot from? Is there a setting or do I need software?
Yes you can, but no software is required. Windows has a boot menu that you can modify, the file is called boot.ini, but its not a file that you can play around with. Its what you can use to have multiple operating systems running. I believe though most Linux operating systems come with Grub Boot Loader, which is also a Boot Menu. Now, when you come to modify the boot.ini to add another operating system, read this:
http://support.microsoft.com/kb/289022. I'm not an expert at this stuff, but thats the file that you use to be able to boot off multiple harddiscs or partitions. I prefer to use Grub, but if you don't have it just use the Windows one.
And to answer your question on a size just for Windows on the partition, mines about 40GB which I keep only Windows and Microsoft products such as the Office Suite, etc. I just leave space on there for temporary files, and also link your My Documents to a different location as well, keeps that off C Drive.