Learn Spanish (Latin America): Hay que comprar un pañal más grande.

Hay que is used to say "one must" or "it is necessary to" in a general sense, without pointing to a specific person. If you wanted to tell someone specifically "you have to buy," you would use tienes que comprar.

We're learning the negative tú commands. How would I change this sentence to tell a friend "Don't buy a bigger diaper"?

You would say No compres un pañal más grande. For regular -ar verbs like comprar, you form the negative tú command by taking the base form and adding the opposite ending, which is -es.

Why is comprar left unconjugated?

Whenever you use the phrase hay que to say "it is necessary to," the action verb that directly follows it must always be in its infinitive (unconjugated) form.

Would más grande change if we were talking about a feminine noun?

No, because grande ends in -e, it is exactly the same for both masculine and feminine nouns. However, it does change for plural nouns. If you needed to buy bigger diapers, you would say unos pañales más grandes.