Max Fellowship (group) size, is 6
- Small Fellowship = 3
- Full Fellowship = 6
- Half-Raid = 12
- Full-Raid = 24
However, do note that some quests, if done without a fellowship when a fellowship is required, will give you Inspiration. Inspiration gives you a massive health boost (300%), a massive power boost (250ish%), +250% damage, +3000 regeneration rate. Basically, it's a "God Mode" because you are practically invincible. Some quests are: Vol 1, Bk 2, "The Red Pass", All of Vol 1, Bk 3. Those are the Inspiration quests I have done. There are many more.
Check the lorebook/lotro-wiki.com for more.
Edit: Please note that Skirmishes, can be scaled from Solo to Half-Raid.
Edit: Also note that Private Instances can be scaled for just the LEVEL, but not all of them, like GA.