In the following sentence
Es ist nur einen Zentimeter groß.
why is it written with Accusative and not Nominative?
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Es ist nur einen Zentimeter groß.
why is it written with Accusative and not Nominative?
This so-called accusative of measure (Akkusativ des Maßes) stands with certain adjectives such as groß, breit, hoch, weit, schwer. Some examples:
Adjectives can also stand with a dative, genitive or a prepositional phrase:
The adjective forms a unit with the dependent phrase that can appear in first position (2.) and prenominally (5.).
In a German sentence, basically any part that isn't subject, object or predicative can be seen as a so called Adverbial. An Adverbial modifies the sentence by adding additional information, similar to how an adverb or adverbial clause in English can.
If you use a noun or a noun phrase as an Adverbial, you can express the way of the modification by the case. For example, an Adverbial des Zeitpunktes (adverbial of point of time) uses the genetive
Das wird euch eines Tages noch leid tun!
The accusative is used, among other uses, for periods of time
Ich habe den ganzen Nachmittag geschlafen.
or, as in your example sentence, the dimension
Es ist nur einen Zentimeter groß.
There are also terms like "adverbialer Genetiv" or "adverbialer Akkusativ".