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> ...., これ以上ないくらい **に** 大きく目を見開いて、...  
>  she opened her eyes as wide as possible ...

I don't understand what に is doing here. I've seen plenty of examples where
くらい acts adverbially without needing に. So why is it there, and what happens
if I omit it?

If it helps, my literal understanding is to parse it as "to the extent that
there is nothing more than this she widley opened her eyes".

I read [this answer](https://japanese.stackexchange.com/questions/35910/is-
the-word-%E3%81%90%E3%82%89%E3%81%84-a-noun-why-do-we-need-%E3%81%AE-
between-%E3%81%90%E3%82%89%E3%81%84-and-%E3%81%AF%E3%81%9A) for particle の but
that doesn't seem to apply here since に cannot (as far as I know) attach to
ない.

Aside: does 目を見開く have some cultural significance e.g. does it represent
concentration for example? The subject is currently beating a dog with a
broom. It says that she made a 一 shape with her lips (seems like something
you'd do if you were concentrating) and then the above sentence. I would
normally associate wide eyes with fear (I don't think she's scared), but that
would be passive. This sentence is active.
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grammar particle-に particle-くらい
Difference between くらい and くらいに
Japanese dictionary "goo" shows the meaning of "位（くらい）" as 3 patterns.

<http://dictionary.goo.ne.jp/jn/63287/meaning/m0u/>

I show you the usage of "に" for those model sentences. (explanations are
ommited. See the original web site of the dictionary.)

１ 「一〇歳位の男の子」「その位で十分だ」

NG: 「一〇歳位に男の子」「その位に十分だ」

OK: 「今はもう、１０歳くらいに（は）なるだろう」、「いい加減にしろ。その位にしておけ」

OK: 「10歳くらいにはなる男の子」、「その位にはしても構わないだろう」

２ 「声も出ない位びっくりした」「犬位人間に忠実な動物はいない」「目に見えない位小さい」

OK: 「声も出ない位にびっくりした」「目に見えない位に小さい」

NG: 「犬位に人間に忠実な動物はいない」

OK: 「犬程に人間に忠実な動物はいない」＝「犬が示す程度に、人間に忠実な動物はいない」

３ 「簡単に否決される位なら、提案しなければよかった」

NG: 「簡単に否決される位になら、提案しなければよかった」

About the dictionary meaning 1st and 2nd, I can say that "位に" indicates the
maximum value or degree that could be acceptable or realizable.
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Both ～くらい and ～くらいに can function adverbially, and I don't see a difference in
meaning between:

> これ以上ない **くらいに** 大きく目を見開いて・・・

and

> これ以上ない **くらい** 大きく目を見開いて・・・

Similar examples:

>   * 子供の **ときに** 京都に引っ越しました。/ 子供の **とき** 京都に引っ越しました。
>   * 子供の **ころに** よく遊んだ公園 / 子供の **ころ** よく遊んだ公園
>   * **すぐに** 伺います。 / **すぐ** 伺います。
>   * **まっすぐに** 立ってください。 / **まっすぐ** 立ってください。
>   * **しっかりと** 持つ / **しっかり** 持つ
>   * **ぼんやりと** 眺める / **ぼんやり** 眺める
>

* * *

> does 目を見開く have some cultural significance e.g. does it represent
> concentration for example?

目を見開く _can_ indicate concentration, but here in your example I think it shows
緊張 (tenseness) and/or 興奮 (agitation/excitement?) It can also be used to
describe one's surprise, anger, fear, admiration, seriousness, etc.

* * *

(By the way, 目を[見張]{みは}る typically represents surprise and/or admiration.)
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I think the meaning of your sentence don't change if you omit に. I guess に is
used to definition an adverbial function or just the emphasis.

I don't feel 目を見開く has some cultural significances. When man angry or when man
are joyful, of course when man feel fear, in various situations, they open
their eyes wide.
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I suppose they are same, because they can be replaced. For example:

>   * 同じくらい(に)
>   * 小指くらい(に)
>

But くらいに is not very used. So I recommend to say くらい.
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