From this article http://tamrielfoundry.com/2012/10/interview-brian-wheeler/ it seems like max group size is 20+, but I wonder if that is a PvP raid group / PvE raid group. They are of course talking about PvP in the article but I don't see why PvP raids would have one max group size and PvE raids another.

The way I got to the number 20+ is from this statement:

TF: Obviously keeps are the primary objectives in Cyrodiil, but there are also supporting buildings and outposts. Can you describe those in a bit more detail?

BW: Yeah, so there are outposts which are smaller versions of keeps, then there are farms and lumber mills and other things that support the keep itself and help feed its upgrade status. The outposts are designed for about 10 people to take, while keeps are designed for 15-20+, while the smaller support structures can be taken by smaller groups 4 to 6 players.

So basically what I am wondering is if the 15 to 20+ part, is a raid group or if you can in fact have that many playes in any group? It would seem excessive to have 20 man groups for anything less then a raid so you can begin to see why I have posted this.

It really depends on the size of the raid that you're taking on, what type of Boss you're facing etc. World Bosses can be incredibly difficult for even a group of maxxed out capped characters. In SWTOR, you see ops groups of 8 and 16, but some require up to 24. So in the trend continues with these types of games, it would really depend on the difficulty of that particular raid.

You probably won't see too many 20 man groups, and there is probably a different setup for these large groups. Most partys would consist of say 4-8 people, and anything larger would require a raid status group. I'm sure you could roll with 15-20 people in the regular game, but it would just be excessive. You wouldn't need that size group until you touch on some end-game content.