I'm waiting on my 6.5 gallon fermenter to arrive. What's the biggest batch I can put in there. Can I do the whole 6.5, or should I leave some space at the top?
Definitely leave some head space or you'll have a mess on your hands (and floor, and walls, and ceiling... armchair covers, throw pillows, and carpeting...). How much head space you need will vary on the yeast and the beer, but most people do 5 gallon batches in them.
The most you should put in there is 5.5 gallons. You could possibly put in more, but then you would need an awfully large diameter blowoff tube. You don't want your airlock/blowoff to get clogged with krauesen and then shoot beer all over your ceiling.