Learn Russian: Столица большая и красивая.

In Russian, the present tense of the verb "to be" (am, is, are) is almost always left out. Because of this, Столица большая literally translates to just "Capital big".

Why do the adjectives большая and красивая both end in -ая?

They are describing the noun столица (capital). Since столица ends in -а, it is a feminine noun. Adjectives must match the gender of the noun they describe, so the dictionary forms большой and красивый take their feminine -ая endings here.

Since a capital is a location, why isn't it in the prepositional case?

We only use the prepositional case when talking about being located in or at a place (usually following the prepositions в or на). In this sentence, the capital is just the main subject being described, not a place someone is currently located in, so it stays in its basic dictionary form (the nominative case).

Would the meaning change if I said Большая и красивая столица?

Yes, word order acts as a big clue when the word "is" is missing! Putting the adjectives before the noun translates to the phrase "A big and beautiful capital", which sounds like an incomplete thought rather than a full sentence.