Learn Italian: È ragionevole dire ciò? - Is it reasonable to say that?

ciò is an invariable pronoun used to refer to abstract ideas, situations, or entire statements (meaning 'this' or 'that'). You could also say quello here, but ciò is slightly more formal and perfectly suits abstract concepts like the one being discussed.

Could I just say dirlo instead of dire ciò?

Yes, you could say È ragionevole dirlo? (Is it reasonable to say it?). Using ciò as a separate word places a slightly stronger emphasis on 'that specific thing' compared to the attached pronoun -lo.

Why isn't there a preposition like di or a before dire?

In the structure 'È + adjective + infinitive' (like È ragionevole dire), the infinitive phrase acts as the subject of the sentence (To say that is reasonable). When the infinitive is the subject, no preposition is needed before it.

Why doesn't the sentence start with a word for 'It'?

Italian almost always drops subject pronouns, including 'it'. For general or impersonal expressions like 'It is reasonable' or 'It is necessary', you simply start directly with the verb È.