"It is wide."
I got this question on the final test, but it was never mentioned during the lessons.
Es means it is. The subject pronoun el or ella might be used if you were clearly discussing a masculine or feminine object, but subject pronouns are most commonly omitted except when used for clarity or emphasis and this is especially true for things. Lo is the masculine direct object pronoun. There is no direct object in the sentence.
That would be this is wide. But they would probably have been looking for esto, the neuter this, had the sentence said this as we have no referent with a gender. Spanish does not have a neuter subject pronoun for "it". If there is a referent with a gender, you refer to the it with the appropriate gender, otherwise you always drop the pronoun.
Big and wide are not the same. Someone with a tiny waist might have a wide belt, but people wouldn't call it big. Length and width are the properties of a two dimensional representation, depth adding the third dimension. If something is wider than it is long or deep you may well call it wide, or even if it were wider than other similar ones. But that thing may still be small.
Both are sometimes translated as wide, but ancho is what we normally consider wide, and the word for width is el ancho. Amplio is more like its English cognate ample. It is generally used meaning wide as in wide open spaces. It is used for clothes to mean loose fitting for some items or full as in a full skirt. If you called a room amplio you would mean spacious, but if you called it ancho you just might be suggesting that it was wide for its length.