You're right. The verb grow can be either transitive or intransitive.
Although # 1 is most common, # 2 is also correct.
1. I grew 5 cm since last year!
2. I grew by 5 cm since last year!
(In #1, was 'by' omitted before '5 cm'? Then, 'grew' was used as an intransitive verb. If not, 'grew' was used as a transitive verb and '5 cm' is the object of 'grew.' Would you let me know about that?)
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Thank you. Then, do you mean in #1, 'grew' was used as a transitive verb and 5 cm is the object of 'grew'?
Right.
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