Learn Spanish (Latin America): Ustedes prefieren un proyecto grande.

Prefieren comes from the verb preferir (to prefer). It is a "stem-changing" verb, which means a vowel in the middle of the word changes when we match it to a person. For preferir, that second e changes to ie in most forms, giving us ustedes prefieren. We don't make this change for nosotros (we), which stays as preferimos.

The verb ending -en is usually for "they". Why is it used here for "you"?

In Latin America, ustedes is the standard way to say "you" to a group of people (like "you all" or "y'all"). Even though it means "you", it always borrows the exact same verb endings as ellos and ellas (they). That is why we use the -en ending here.

Why does grande come after proyecto instead of before it?

In Spanish, describing words (adjectives) almost always go after the thing they are describing (the noun). So instead of "a big project" like we say in English, the order is un proyecto grande (a project big).

Would grande change to "granda" if it were describing a feminine word?