[dnanian]

I'm not sure that SD! is going to help with compulsive installing, actually: you'll want to update your main drive sometimes, and that'll end up bringing your Sandboxed applications over...

But, anyway, the size of the Sandbox can be determined by using a "too big" volume first. Create a Sandbox on it, and check its size. Then, add about 20% or so for regular use, more if you're going to install a lot. And there you go -- that' sthe size you need.

You should always continue to back up your main drive.

Time Machine should still be backing up your main volume and the Sandbox volume (if desired).