> ある事柄について感じた事(が、発表できる程度 **に** まとまったもの)。

So, I understand the first sentence, but I'm a little confused about the
second one.

My translation is: to have a feeling about a certain matter to the extent of
being able to declare about it.
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usage particles particle-に
What is the meaning/usage of に in the following sentence?
My understanding is that both parts of your sentence describe the `もの`
mentioned at the very end of the sentence.

> ある事柄について感じた事がまとまったもの

A thing that collects the things felt about a certain matter.

> 発表できる程度にまとまったもの

A thing that collects (something) to the degree of being able to present (it).
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The simple answer: the に in 発表できる程度にまとまったもの allows the adverb of degree
程度(extent) to modify まとまった.

発表できる程度 translates to "the extent it can be presented".

まとまる in this context translates to "to come together; to cohere".

In order to say that ある事柄について感じた事 has まとまった to 発表できる程度, we use に.

So:

> ある事柄について感じた事(が、発表できる程度にまとまったもの)。

becomes, in English:

> (The substance of) what you felt about a certain matter (that has become
> coherent enough to be able to be presented (outwardly)).
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Isn't this an excerpt of a dictionary definition, like what [this blog
cites](http://blog.livedoor.jp/bokuryu/archives/10319966.html)?

> 「感想」…ある事柄について感じた事（が、発表出来る程度にまとまったもの）。

In this case, it means:

> what one thinks/feels about a certain matter (which has taken shape enough
> to be expressed/presented/published)

As you have correctly translated, 程度 means "(to) the extent/degree", or you
could take Vできる程度 altogether as "enough to V". Anyway, both ～程度 and ～程度に
stands for the adverbial phrase "to the degree", but the form with に is
preferred when describing the state how the result is, while without に the
manner how the action is done.

> 30cm程度切る _cut (off) 30cm_  
>  30cm程度に切る _cut into 30cm pieces_ or _trim into 30cm long_
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