Learn Spanish (Spain): La plaza es muy grande y bonita. - The square is very big and pretty.

We use es (from the verb ser) because we are describing the fundamental, physical characteristics of the square. While both mean is, Spanish uses ser for permanent descriptions and estar for temporary states or locations.

Since la plaza is feminine, why doesn't grande end in an a?

Adjectives that end in the letter -e in Spanish, like grande, do not change for gender. They stay exactly the same whether they describe a masculine noun or a feminine noun like la plaza.

Why does bonita end in an a then?

The base word for pretty is bonito, which ends in an -o. Adjectives ending in -o must change to end in -a when describing a feminine word. Because la plaza is feminine, bonito becomes bonita.

Do I need to repeat muy before bonita?

No, just like in English, you only need to use muy (very) once. It naturally applies to both grande and bonita. You could say muy grande y muy bonita for extra emphasis, but it is not required.