1. I've always used 1/2 cup DME to 1 pint water. I pulled that from the Palmer book, and it's always worked well for me. Go to www.mrmalty.com, enter the specifics for your recipe, and it will tell you how large of a starter you need. Once you've done that, just build up the starter to the recommended volume.
2. Is the carboy your primary? I don't see anything wrong with making your starter in there, per se, but you'll want to be able to swirl the yeast around to aerate them. I'm sure you "can" swirl a carboy, but if it's the glass kind it's going to be a pain. Personally, I use a 2L erlenmeyer flask for my starters, but you can use anything as long as it's sanitized well. Look for juice bottles, etc. that have an opening the same diameter as your bung.