the AOE is against those in a certain Radius. I believe it might cap at like 10 players but have not proof of this as .... non of us were inside that area save a Tank. You need atleast 2 Healers for the MT and the MT potting to do this boss. After downing him once I have a strong feeling that if we had the right group we could 6 man the Leeching Spider.
We might be able to do it with a 6 man one day.... but I doubt that day will come anytime soon, I say its doable with 12 ppl at least. Both of these bosses change mechanics after 50%, and everything gets a little crazy
I hope I didn't reveal too much info about these fights, then again its not like any other guild has the numbers to attempt them yet, we even had to borrow players from other guilds... guild coop ftw!