Learn Polish: Moja ręka jest duża.

In Polish, words for 'my' have to match the gender of the noun they describe. Because ręka (hand) ends in an 'a', it is a feminine noun. Therefore, we use the feminine form moja instead of the masculine form mój.

Why does the word for 'big' end in an 'a' (duża)?

Just like moja, describing words (adjectives) must match the gender of the noun. Since ręka is feminine, the adjective takes the feminine ending -a, making it duża instead of the masculine duży.

I've heard ręka used for 'arm'. Does it mean 'hand' or 'arm'?

It actually means both! In everyday Polish, ręka is used to talk about the entire arm from the shoulder to the fingertips, as well as just the hand. You can usually tell which one is meant from the context.

Do I always need to include jest in a sentence like this?

Yes. Polish requires the verb 'to be' (być) in these kinds of descriptive sentences. You cannot just drop it and say Moja ręka duża — the word jest (is) is completely necessary to connect the subject and the adjective.