1) How big should I make the partition? Should it be FAT32 or NTFS? Does it matter? Am I just worrying too much and should I just format the damn thing and let the Mandrake installer guide me through it?
You should let the Mandrake Installer do it, but you won't be formating to NTFS or FAT32, but rather either ext2 or ReiserFS. Again, this should be done automatically.
2) Do I have to modify the C:\boot.ini file myself, or will Windows handle it?
No, it should install either LILO or Grub (whichever you choose, I think Grub is the default) and make the proper entried.
3) Can I store my Linux programs on my primary drive?
Maybe, but I wouldn't suggest it. You'll have to mount that drive for your Linux install to see anything on it.
4) Will the installer come with drivers for my videocard and printer, or do I have to find and install them manually?
Again, maybe. Most vid cards are there, but some are not, so read the documentation and ask questions because there might be some issues or special needs there.
In all honesty, if HD space is an issue, I'd probably be looking at something like Mandrake move or using the Mandrake LiveCD instead if you're not looking at adding a new HD or clearing out some space for it.