Learn Spanish (Latin America): Ella tiene un cuello largo.

No, largo is a false friend. It looks like the English word large, but it actually means long. If you want to say large or big, you use the word grande.

Why does largo come after cuello?

In Spanish, descriptive adjectives like largo (long) usually come after the noun they describe. You literally say a neck long (un cuello largo).

Since the current lesson is about verbs like gustar, how would I say Her neck hurts using this vocabulary?

To talk about pain, you use the verb doler, which works just like gustar. You would say A ella le duele el cuello (literally: To her, the neck causes pain).

Why do we use un (a) here instead of el (the)? I thought body parts use the!

Good catch! When you just talk about a body part itself (like it hurting), Spanish uses el or la (the). But when you describe what kind of feature someone has using an adjective, like having a long neck, you use un or una just like in English.