Can skinny people feel almost all their normal lymph nodes in neck groin and armpits? what size is considered unenlarged?

Sort of: Normal lymph nodes are much more palpable on thin people. If there is any question, let your doctor check it.

size differs: 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 in response to the inflammation they create when they react. They shrink when all is well.