I've been told that it's normal for me to be able to feel nodes in my neck and groin since i am very slim and only 16. i've had the nodes for 2 months and there pea sized. but how can i tell when the nodes are enlarged if there always pea sized?

Pea sized is normal. Normal is and will be normal. When nodes are stimulated by the need to clean body serum they enlarge to grape size or bigger. Rarely they can become infected by the germs they process and need to be drained. At that point they are obviously swollen/painful & change the color of nearby skin.

You may feel the nodes every couple of weeks, if any of the lymph glands doubles in size, becomes painful, ulcerates, or other nodes become enlarged, or you develop fever, or lose weight, you should see a doctor. Wish you good health!