What is Wendy talking about when she says that Tink can’t go a twentieth part of the way round? The way round what?
I think it means that if Tinkerbell was sat around the fire with Peter, she wouldn’t take up even 1/20 of the circumference, so there would still be a lot of empty space around the fire.
Here’s some other people that are equally confused about the line. I was going to try to explain what they were saying, but it’s probably easiest it I just link to it.
Fairies (in English folklore) don’t live very long. When it comes time for Wendy’s first annual return, Tinker Bell has died and Peter’s forgotten her.
Yes, but I don’t think that’s what the line means, because Wendy is surprised that Tink is no longer around and asks Peter where she is. I get the impression that whatever the line means it’s something embarrassing for Tink because she calls Wendy a tattletale for saying it.
I already googled the phrase in the hopes of finding something. 
Those explanations are interesting but none of them seem right to me. Oh well.
Then I’d probably just go with the 20th of the way around the fire explanation. She’s so tiny, that it’ll still feel lonely and empty and it makes sense in context. The only part that doesn’t make sense is why Tink yelled out the comment, but I haven’t read the book, so maybe that’s in line with her character.
And, yes, the people on the other board got a little overly philosophical right off the bat.
That Tinkerbell doesn’t take up 1/20 the circumference of the fire doesn’t make sense, either, because Tinkerbell calls Wendy a “sneaky tell-tale.” That implies Peter wouldn’t have known were it not for Wendy. *Obviously *Tinkerbell is small.
“Can’t go a twentieth part of the way round” is at least something Peter wouldn’t know, or at least not without some reflection.
I think it’s in character for Tinker Bell not to realize that it’s obvious how small she is.
Oh. Yeah, I agree with that.
What about the interpretation that Tinkerbell can’t hug him?
Based on the language, it would have to not only be embarrassing, but something that Peter already knows, but Tinkerbell does not know he knows. Wendy “reminds” Peter of this fact, but you can’t be a “tell tale” unless you’re telling someone something they don’t know.
And the fire explanation makes no sense for an even bigger reason. Wendy also would leave a lot of space around the fire.
The context, according the link above, is Peter talking about where he’s going to live. So I like the housework explanation. It makes sense that Wendy is saying the Tink will not be able to take care of Peter like Wendy could. It fits the gender roles of the time, and the theme about Wendy wanting to be an adult.
And of course Tinkerbell is embarrassed about this, and it is in character for her to think that Peter wouldn’t have figured that out, for the same reason she’d think he wouldn’t figure out how small she is.
I don’t think there is a literal interpretation of this. Just comes down to Wendy meaning that Tinkerbell is not adequate. Her presence won’t make up for Wendy’s absence and all that Wendy brought to the household. It wasn’t a household as such back when Tink was the only female, and now it will revert to a bachelor pad. As for “sneaky tell-tale,” well, Tinkerbell never liked Wendy. She would have said the first thing that came into her head, as long as it was negative. Didn’t have to be a direct response.
Barrie made a lot of allegorical comments; I mean, the whole story is an allegory, after all. In the same chapter being quoted here:
And there’s Mrs. Darling’s “sweet mocking mouth” with the one kiss no one could ever get, “though there is was, perfectly conspicuous in the right-hand corner.” Until Peter leaves and takes it with him. And probably puts it somewhere and forgets about it. Point is, Barrie had a unique voice. Don’t try too hard to understand, or you’ll be like the Lost Boys after they’d been in school for a while. “In time they could not even fly after their hats. Want of practice, they called it; but what it really meant was that they no longer believed.”
I bet its sex related
J.M. Barrie hardly had a sex drive.
This was my first thought. Say, a parent and child are hugging. I can understand the kid saying something like “I can’t go all the way around” or “I can only go halfway around.” Wendy is reminding Peter that if he’s lonely, he can get only one-twentieth of a hug from Tink. Is it possible that this phrase was well-known in early 20th-century Britain?
Hey, I think you guys are right about the hugging! I found the text of an earlier version of the play where Tinkerbell was called Tippytoes:
Based on the earlier script it does seem like the phrase means Tink can’t properly hug anyone. Anyway, thanks everyone.