When it comes to lymph nodes how big is bad. like lymphoma?

Depends: It really depends on location. On ct scans, some nodes over 8 mm are considered abnormal, some need to be over 10 mm. However, in some cases, in certain areas, just seeing lymph nodes is abnormal. Also, sometimes we see too many nodes, even though they are not enlarged, and that is abnormal.

Lymphadenopathy: Lymph nodes enlarge as a result of their normal activity in aiding in immunity, but may also enlarge because of malignancy of the lymphatics or extension of other cancers to the lymph nodes. When cancerous they are often hard to touch, fixed to underlying tissues, not tender. The best answer however, is that if concerned, they may need to be biopsied for a diagnosis.