Learn Spanish (Latin America): Esa frase es muy larga.

To report a statement made in the past (using a reporting verb like dijo), you step the original verb back in time. The original verb here is the present tense es. In reported speech, the present indicative changes to the imperfect. So, 'He told me that that sentence was very long' becomes: Me dijo que esa frase era muy larga.

Why do we use es instead of está for 'is very long'?

We use the verb ser (es) because length is considered an inherent characteristic or physical property of this particular sentence, not a temporary state. When describing the fundamental traits of a thing, we always use ser.

Why is the word for 'that' esa and not eso?

Esa is a demonstrative adjective modifying the feminine, singular noun frase. Eso is the neuter form of that, which is only used as a standalone pronoun for abstract ideas, statements, or unknown objects (like asking 'What is that?'). You never put eso directly in front of a noun.

Is there a difference between frase and oración?