Both are fine.
"it is not big" could be tInbe' or tInbe' 'oH.
Since the verb prefix identifies the subject and the object, pronouns are often not necessary and can be left out or put in as you wish.
Though with third-person subjects or objects, using a pronoun can be useful to distinguish between ghaH "he, she, him, her" on the one hand and 'oH "it" on the other, or between chaH "they, them" (plural of ghaH) on the one hand and bIH "they, them" (plural of 'oH) on the other.
So tInbe' could mean not only "it is not big" but also "he is not big, she is not big, they are not big". tInbe' 'oH can only mean "it is not big".
tIn means "to be big". -be' is a verb suffix which makes the sentence negative.
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