My sternocleidomastoid muscle on the right side of my neck is enlarged. feels a bit stiff. no tenderness or pain. noticeable larger than left side. ?

Muscle: If this muscle is elarged you might have a myopathy and the muscle may need to be biopsied before a diagnosis can be made

Probably spasm: The stiff Muscle can be inflammed and feels swollen. Try to apply Moist heat/Ice. Use Anti inflammtory medicine like Aleve (naproxen) or Advil . May apply topical Muscle relaxant. If not better in 3 days Get examined.