For pepper spray I carry a 2oz can (bigger than the keychain sizes but still much smaller than the "bear mace" sized cans). I originally started carrying it for defense from dogs (I used to be a delivery driver) but also figure I could use it in a defense situation before lethal force becomes necessary.
As far as which brand or whatever is better I have no opinions.
I bought mine from http://www.pepper-spray-store.com/ and they have quite the selection of pepper spray products and the prices seem reasonable enough. I haven't done any comparison shopping however.