I assume it's just an arbitrary choice in this case. Whatever "it" refers to here can be assumed to be a feminine noun. I think if you were translating the English phrase into French, either "il" or "elle" would be fine.
elle might depict an island or a beach since it is feminine: la île, la plage.
I have a hard time to translate sentences with measures in french. Is it right to say that the sentence "It is four kilometers long" can be translated in three different ways (assuming the subject is feminine):
Elle est longue de quatre kilometres
Elle fait quatre kilometres de long
Elle fait quatre kilometres de longueur
That seems to be correct.
The first link above suggests that WillB_OU is incorrect in saying that the noun needs to be present for your last two examples. I can't vouch for its accuracy, but it gives the following:
How long are your skis?
- De quelle longueur sont tes skis ?
- Quelle est la longueur de tes skis ?
- Combien mesurent tes skis ?
- Combien font tes skis de long ?
They are 2 meters long.
- Ils sont longs de 2 mètres.
- Ils ont 2 mètres de long.
- Ils mesurent 2 m.
- Ils font 2 m.
You use etre when the noun of what you're measuring it absent:
it is 7 kilometres long= elle est longue de sept kilomètres
When you use the noun of the object measure you use:
L’île fait environ cinq kilomètres de long et deux de large= the island is about 5 kilometres long and 2 kilometres wide. (an example from this lesson).
So your last 2 examples above are incorrect.
Well if that's so, how come the first two can't be back-translated as "it is four kilometers in length?
I don't see why they couldn't be, though the first one clearly uses an adjective, "longue", so it's arguably a better reflection of the French grammar to use "long" instead of "in length" for that one in English.
It seems to me that "It has a length of 4 kilometers" is equivalent, but DL doesn't agree. In any case, I reported it.
The problem is that the latitude in how literal you need to be is totally arbitrary from one sentence to another. Sometimes (but not always) there is a well-known equivalent expression in English and they allow (or require) it, even though it's not literally equivalent. You can never count on it, though.
I reported it as well. As I understand it, the problem is that DL is database driven. So each correct answer has to be entered into the database by hand or it won't be accepted. And since the best moderators are native French speakers, they don't always think of the most obvious English phrasing. And then, sometimes they are even trying to make a grammatical point and insist on a particular phrasing to show that you understand the grammar, often forcing an awkward English sentence. If we keep reporting these, they usually get the message after a few years.
Its length is 4 kilometers. It has a length of 4 kilometers. It is 4 kilometers in length. it's 4 kilometers long.
All are correct translations that a UN translator would jump at in a second even though the grammar differs subtly. Here, it takes some argument depending on the moderator's views.