the-elder-scrolls-online
Our ESO group is looking at tackling Dragonstar Arena in Craglorn.
But the wiki says it's "aimed at groups of four players rather than twelve".
Does that mean it's limited to 4 players like a dungeon & we can't play as a group of 5 or 6? What is the maximum group size for Dragonstar Arena?
You can indeed, it is under the Gameplay options as seen in this image here: 
I'm gonna give my answer based off of experience.
Each boss or dungeon (not sure) has unique items, how many unique items I couldn't tell you, that drop from bosses. They also drop the generic gear, for example "Leather belt of stamina". You are not always garunteed to get good items from a boss.
I have gotten everything from nothing from an entire dungeon to a couple epics in a dungeon, it is purely luck.
Also something to note, every elite boss in ESO has a unique item. Atleast that I have come across.
Best Answer
The game basically knows three sizes for its end game PvE "raid" content,so called trials:
Regular trials are challenges for up to 12 players:
Dragonstar Arena is a special 4 player trial you can find in Craglorn.
Mahlstrom Arena is a special solo trial you can find in Wrothgar (requires Orsinium DLC).