insist the Vet take a needle aspirate and send it off to pathology ASAP. If it's Lymphoma then you need to confirm it quickly. Avoid all Prednisone in the mean time while awaiting results.

The appearance of Demodex if that is the case is suspicious. Other typical signs of Lymphoma include: elevated temperature (i.e. secondary infection), anorexia, lethargy, weakness in walking or climbing stairs, muscle wasting, and GI upsets. Some Lymphoma puppies can also be almost asymptomatic when diagnosed.

The size micki gave is typical, but Lymphoma can affect each dog differently.

Hopefully it will be swelling due to infection or even the mites.