I recently had a Y which was comprised of 27 hexes, and which hadn't in any way been artificially extended. I don't remember seeing any larger structures so far - is it somewhat unusual?

What's the largest successful loop? (Either perimiter or area.)

I think this is my largest loop. Perimiter 13, enclosing 7 cells. I'm sure there have been larger.

16 cells surround 12

27 is quite large. Glancing at a bunch of my games, the largest Y I might have formed (if my opponent hadn't resigned) would have been 'only' 26 hexes.

A fork of 22 is my largest win and of course my largest fork that I lost is Fatphil's famous 27!

What about the game with the most moves, I had one with 71!

My current longest game so far is 93 total moves.

Which also has 27 hexes in its winning Y, Ed.

Ah, so it does. I must have missed that game.

Have there been any winning Y structures (a fork) where none of the three sides are adjacent to each other?

97 moves gid=962718

Freeling and I had a game with 103 moves! I think it's the longest.

Game #1021161 ended with 108 moves.

now we should distinguish between size 8 and size 10?

A size 10 game in 129 moves and a nice puzzle:

Black to win:

[game;id:1009271;move:98;title:Black to play and win]

Well, back onto largest winning structures - here's a monster:

[game;id:1009256] Perimeter 25, contents 21, if I've not miscounted.

I think this one's as large as the previous one.

[game;id:1074859]

I think this one was fairly big, though I suppose the outcome had been clear for some moves:

[game;id:1018032]

Oh, and I mean the number of moves, not the actual winning structure, which is of moderate size.

But it's a huge connection too!

Quite a large black structure.

[game;id:1131998]

Note, that all the black stones (maybe except R4) form a single frame, connecting four borders and one corner.

However, babyruth was deadly wrong with move 56

[game;id:1131998;move:56]

Here F12 wins…