Learn Russian: Всё хорошо в меру. - Everything is good in measure.

Всё means "everything" and is treated as a neuter singular pronoun. That is why it pairs with the neuter word хорошо. The word Все means "everyone" or "all" and is plural.

Why is хорошо used instead of the full adjective хорошее?

In Russian, when making a general statement that a subject "is" a certain way (like "everything is good"), you usually use the short form of the adjective. Хорошо is the neuter short form, perfectly matching the neuter subject Всё.

Why is the word мера (measure) in the accusative case (меру) in this sentence?

The phrase в меру is a fixed idiom that translates to "in moderation." The preposition в combined with the accusative case is often used to indicate a boundary, limit, or extent, meaning you should do things up to a healthy measure.