I'm not surprised he became an author. Even as a child he had a...

[Volcano1985]

I'm not surprised he became an author. Even as a child he had a ... imagination

large
great
vivid
bright
clean

I think vivid or bright

[Milanya]

vivid
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con·text - The part of a text or statement that surrounds a particular word or passage and determines its meaning.

[Sayyora]

i think ' as a child he had a great imagination'

[Anglo Sax]

Sayyora, this is one of those subtle ones, 'vivid imagination' is correct. It's one you just have to learn. Great could mean wonderful, but 'vivid' suggests realism with a lot of detail and is generally used in this context. The concept is reinforced by 'even' as a child... suggesting it was more vivid as an adult, enabling him to become an author.

[Cerberus™]

I'd say "vivid imagination" is a pair of words that are found together so often that one immediately triggers the thought of the other. It comes naturally to use them together if possible. Using one of the other words would not be completely wrong, but a bit odd, since it would be contrary to expectation.