Learn Hardcore Japanese: hiroi heya de kaigi wo simasu. - We will have a meeting in a spacious room.

Action location: 広い部屋で会議をします。 (We will hold a meeting in a spacious room.)
Existence: 部屋に机があります。 (There is a desk in the room.)
Events are also typically at a location with で: 教室で授業があります。 (There’s a class in the classroom.) For destinations, use へ/に: 会議へ行きます。 (I’m going to the meeting.)

What is the role of を after 会議? Is it necessary?

In 会議をします, 会議 is the direct object of する (to do). Many する-nouns work this way. The を is often optional in speech:

会議をします／会議します／会議をする／会議する all mean “have a meeting.” In formal writing, 会議をします or alternatives like 行います are more common.

Does 広い modify 部屋? How do adjectives before nouns work?

Yes. 広い is an i-adjective directly modifying 部屋: 広い部屋 = “a spacious room.”
When an i-adjective is the sentence predicate, you use it at the end:

部屋は広いです。 (The room is spacious.) No copula is used between the adjective and the noun it modifies.

Who is the subject? There’s no I/we/they in the sentence.

Japanese often omits subjects when they’re clear from context. 会議をします could mean “I/we/they will have a meeting.” To specify:

私たちは広い部屋で会議をします。 (We will have a meeting in a spacious room.)
部長が広い部屋で会議をします。 (The manager will have a meeting in a spacious room.) Use は to set a topic, が to mark the grammatical subject.

Does します mean present or future?

It’s non-past and can mean habitual present or planned future. Context/time words clarify:

Can I change the word order?

Yes, as long as particles stay with their words and the verb remains at the end:

広い部屋で会議をします。 (neutral)
会議を広い部屋でします。 (slight emphasis on the location right before the verb)
広い部屋で会議します。 (を dropped; casual to neutral) Avoid moving the verb away from the end: × します会議を is wrong.

Could I use に or へ after 会議?

With different verbs, yes:

会議に出ます／参加します。 (attend a meeting)
会議へ行きます。 (go to a meeting)
会議にします。 (decide to make it a meeting; choose “meeting” as the option) But to “hold/do” a meeting, use 会議をします (or alternatives like 開きます／行います).

What’s the difference between 広い and 大きい for rooms?

広い emphasizes spaciousness (plenty of room to move around).
大きい emphasizes overall size or dimensions.
Both can modify rooms, but the nuance differs:
広い部屋 = a roomy/spacious room.
大きい部屋／大きな部屋 = a big/large room (common phrasing is 大きな部屋).

Why isn’t there です after 広い here?

Because 広い is attributive (modifying 部屋). Use です when the adjective is the predicate:

Is 会議します as natural as 会議をします?

Both are natural. Many する-nouns allow dropping を without changing the meaning:

勉強(を)します, 運動(を)します, 練習(を)します, 会議(を)します In careful or formal writing, keeping を (or using 行います/開きます) is often preferred.

How do I make it plain, more polite, or very formal?

How do I say it in the past or negative?

How do time expressions fit into the sentence?

Place time before the rest. Use に for specific clock times and dates; it’s often omitted for broad times like “tomorrow.”

What does では do in 広い部屋では会議をします?

では (＝で + topic marker は) sets a contrastive topic:

広い部屋では会議をします。 (As for spacious rooms, we do have meetings—implying contrast with other places.)
広い部屋では会議をしません。小さい部屋でします。 (Contrast made explicit.)

Are there alternatives to します that sound more official?

Yes:

会議を開きます。 (hold/host a meeting)
会議を行います。 (conduct a meeting; formal)
会議を実施します。 (carry out a meeting; very formal/administrative) All take を like します.

Does で ever mean “with/by”? How do I tell which で it is?

Yes, で also marks means/instrument. Context decides:

Why are there spaces between the words? I thought Japanese doesn’t use spaces.

Standard Japanese is written without spaces. The normal writing is 広い部屋で会議をします。 Spaces are added in learning materials to show word boundaries.

How do you pronounce the sentence?