"The chair is small."
kahless62003's answer is good! I'm going to say the same thing in another way.
Using a pronoun (like 'oH) as a verb to equate two nouns (this thing is that thing) is only for nouns. Your sentence doesn't work because if you are using 'oH as a verb, you have two verbs and if you are using 'oH as a pronoun, you have two subjects.
There are no adjectives in Klingon. The words that are usually adjectives in English (like "small") are often verbs in Klingon (like mach). So, you just use the verb as a verb and you don't need to use a pronoun as the verb.