How big should the swollen lymph node be if it was affected by melanoma?

Not possible to say!: Lymph nodes are enlarged if they can be palpated during a clinical exam. Lymph nodes can be enlarged for various reasons including infection, lymphoma and from cancer that are metastatic. The size of the node does not tell you anything about the cause. If there is any concern about a large lymph node one should get it checked by a physician. If needed a needle biopsy can be done to sample the node.

Might not be swollen: A lymph node that has metastatic melanoma may be the same size as a normal lymph node.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge the same in response to the inflammation they create when they react regardless of the cause.