How much space to allocate for Win 7

You can't install or boot Windows from an external.

Works fine for Mac OS.

You can leave whatever you want, or none, for OS X but to make it small and easy, clone OS X to an external volume or disk image and then format (erase) and restore Mac OS. Then make Windows to any size.

.... You can't run install or boot Windows off any type of external hard drive from a Mac.

You don't need Mac OS, and it would be easier to dedicate 100% to Windows --- on your internal hard drive.

There shouldn't be any need to concern with how much space to leave unallocated.

".... whenever I needed to use Windows."

"I plan to install Win 7 on bootcamp on an external 500 GB drive with OS 10.6

"How much space should I choose when setting up boot camp?
"I plan to just have bootcamp/Win 7 on that OS 10.6 drive, and otherwise not use it for mac stuff."

The "on boot camp" translates as the way Mac users tend to think of installing Windows to run natively.

You don't "install Snow Leopard on Disk Utility"; Boot Camp (Assistant) is merely a way to allow and add MBR to the Apple GUID initialization.

If you just use Windows, you can dispense with GUID (GPT) entirely and with it Boot Camp or Boot Camp Assistant.

Once Windows 7 is installed you of course need the motherboard chipset drivers just like normal to enable various components.

Rule of thumb for HFS+: never let the system volume get to less than 20% or 40GB free (more is better).

The smaller Windows is, the slower the region of the tracks on the platter.

Your comments left me with the impression you wanted primarily to run Windows, and NOT Mac OS.

I plan to just have bootcamp/Win 7 on that OS 10.6 drive, and *otherwise not use it* for mac stuff.

Mac81 wrote:
This apple site here seems to say that you can install Win on bootcamp on a remote drive with OS X on it:

http://support.apple.com/kb/HT3777

Paul

Here is quote from the document you linked to:

"I have more than one hard drive, can I install Microsoft Windows on any drive?

You can use Boot Camp to install Windows on any internal hard drive, but not on an external hard drive."