Площадь участка измерили.

It is the direct object, and it is in the accusative.

The important point is that for площадь (a feminine inanimate noun of the third declension), the nominative singular and accusative singular look the same:

• nominative: площадь
• accusative: площадь

So even though the form does not change, its role in the sentence is still direct object.

For comparison:

• genitive: площади
• dative: площади
• instrumental: площадью

Because участка depends on площадь and means of the plot / of the parcel / of the site.

So:

• площадь участка = the area of the plot
• literally: area of plot

This is a very common Russian pattern: one noun is followed by another noun in the genitive to show possession or relation.

Similar examples:

• центр города = the center of the city
• крыша дома = the roof of the house
• номер телефона = the phone number

Russian often leaves out the subject when it is unknown, unimportant, or obvious from context.

Here, измерили is a past tense plural form used in an indefinite-personal way. It means something like:

• they measured
• someone measured
• the area was measured

So the sentence does not tell us exactly who did the measuring. The focus is on the fact that the action was completed.

Because Russian commonly uses the third person plural past to talk about an unspecified group of people.

So измерили literally looks like they measured, but it often really means someone measured it or it was measured.

This is a very common structure in Russian:

• Мне сказали... = I was told... / They told me...
• Дом построили быстро. = The house was built quickly.
• Документы уже проверили. = The documents have already been checked.

Grammatically, it is active:

• измерили = active past tense verb

But in meaning, English often translates this kind of sentence with a passive:

• The area of the plot was measured.

So this is one of those cases where Russian uses an active-looking form, but English may prefer a passive translation.

A more explicitly passive Russian version would be:

• Площадь участка была измерена.

That version sounds more formal or official.

Измерили is the past tense of the perfective verb измерить.

The perfective aspect shows the action as completed and as having a result. In this sentence, the point is not the measuring process, but the fact that the measuring was successfully done.

So:

• измерили = measured / finished measuring

This is very natural when reporting a completed action.

Измеряли is the imperfective past tense.

Compare:

• Площадь участка измерили.
  The measuring was completed; the result matters.

• Площадь участка измеряли.
  They were measuring it / used to measure it / measured it as a process.

With измеряли, the sentence focuses more on the activity itself, not necessarily on completion.

So the difference is mainly:

• измерили = completed action
• измеряли = process, repetition, or no clear result

Russian word order is flexible, and changing it often changes the information structure rather than the basic meaning.

• Измерили площадь участка.
  More neutral, especially as a plain statement of what happened.

• Площадь участка измерили.
  Puts площадь участка first, so it sounds more like the topic or something already under discussion.

This fronting can suggest:

• As for the area of the plot, they measured it
• or a slight contrast, depending on context

So the sentence you have is not wrong or strange at all; it just gives a bit more prominence to площадь участка.

Yes, you could say that, but it is a different style.

• Площадь участка измерили.
  Natural, everyday Russian; indefinite-personal; often preferred in normal speech.

• Площадь участка была измерена.
  More formal, written, official, or report-like.

Russian often prefers the indefinite-personal construction where English might use a passive. So измерили is usually the more conversational and idiomatic choice.

The standard stress is:

• плóщадь
• учáстка
• измéрили

So the full sentence is pronounced:

Плóщадь учáстка измéрили.

Stress matters in Russian, so it is good to learn each word with its stress from the beginning.