1. A fresh install of Windows will take up a bit under 2GB of space. If you strip out a bunch of components you aren't going to use, you can probably get it down to 1.5GB or less.
2. Apple recommends 10GB so that you can install various applications and have enough space left over for saving files from those programs. I'm not sure of the typical file size that programs like AutoCAD and Solidworks make, but if they're big and you plan on working on several projects, you might want to go with more than 10GB. Of course, if you ever find yourself out of space, you can always run the Boot Camp Assistant and just increase the partition size.
3. Boot Camp is currently in beta and is scheduled to expire once the final version is released with Leopard. However, if you read the EULA, it doesn't technically expire till September 2007, so you don't have to worry about your copy of Windows not booting up once the final version is released.
4. Intel's Core2Duo line of processors is 64-bit, but I believe only the 32-bit version of Windows XP is "officially" supported because Apple does not provide 64-bit drivers for Windows x64. So you could probably install the 64-bit version of Windows, but it'll be a pain to use without proper drivers, especially for the trackpad.