Esto es / Este es / Esta es

[createdamadman]

So I know that "esto" is the neutral form of este/esta, but I have the following sentences from Rosetta Stone, and I'm trying to figure out if "esto" must be used or if it can be replaced with "este/esta":

"Esto es una pizza entera."


"Esto es un tercio de una pizza."


"Esto es la mitad de una pizza."

Could I just as well say the following?:

"Esta es una pizza entera."


"Este es un tercio de una pizza."


"Esta es la mitad de una pizza."

[AngelicaDeAlquezar]

In this case, they can be interchanged.
It depends on what the speaker is thinking about. If they have a more abstract idea of what they're showing, they'll use "esto", but if their conversation partner already has more context or knows what the speaker is talking about, then they'll say "este"/"esta".

[Rusty]

The neuter pronoun refers to a concept, idea, something already talked about. It doesn't refer to a specific noun.
In your sentences, the demonstrative pronoun appears before the concept (found in the subject complement (predicate noun)). Since the demonstrative doesn't have an antecedent, it must be used in neuter form.

This answers your question, but it doesn't mean that everyone agrees. Generally, if you're not talking about a specific noun (one already mentioned, one you're pointing at, or one that's inferred), the neuter pronoun is usually used.