Learn Italian: Compreresti una barca molto grande?

Compreresti is the "you" (singular) form of the present conditional. For regular -are verbs like comprare, you change the -are ending to -er- to get the stem (comprer-), and then add the conditional ending -esti. This translates to "would you buy".

Why is it grande and not granda since barca is a feminine noun?

In Italian, adjectives that end in -e in their singular dictionary form (like grande) are used for both masculine and feminine nouns. They only change to -i for the plural (grandi).

Does molto need to match the feminine noun barca?

No, because here molto is acting as an adverb meaning "very" to describe how big the boat is. When molto means "very", it never changes its -o ending. It only changes endings to match a noun when it means "many" or "a lot of".

Is it wrong to drop the subject pronoun tu before compreresti?

It is perfectly normal to drop it! The verb ending -esti already makes it 100% clear that you are asking "you" (singular). Adding tu is only necessary if you want to emphasize who you are asking.