I will admit I haven't done much research to see if raiding ( bunch of people who kill bosses in large instances to get high end loot ) will be one of the main end game focuses or not. If you do know, please feel free to share that info here.

If you could pick what size you would want your raid group to be, what number would you go with?

Do you enjoy the controlled chaos of large raids that have 40+ people? Or do you enjoy tight nit groups that have 10 or less people?

I enjoy smaller groups. Much easier IMO to work around player schedules.
The number I would pick for my personal preferred raid size would be 5.

The days of 40+ are long gone, I hope. That's way too much for PvE content, though it might be good for PvP.

For a good PvE raid, 8 is plenty. 12 is OK. 16 is big. Anything bigger than that becomes too much like work between organization, teaching, logistics, and everything else.

5 isn't a raid. It's a group.

I think this is one of the (few) things SWTOR got right - group and raid sizing. 4 to a group, 8 or 16 to a raid.

I hate large raids of 40+ because it takes forever to get everyone assembled. Its not so bad with things like Relic/Elder Scroll raids because in that case you go with what you have and there is no required number of players to be able to enter the dungeon. I hate PvE raids that require a certain number of players to me that is bad game design because if I can come up with a group that can do that dungeon with less players then we should all be rewarded for that and not have to wait for 40 players to show up. Nothing worse then standing around for a few hours waiting and finally you get 40 there and suddenly someone lags out or has to log off...I hated that.