The optimal header size generally depends on the context and design of the document or web page. However, there are general guidelines: H1: 24-32 pixels (1.5-2 em) H2: 20-28 pixels (1.25-1.75 em) H3: 16-24 pixels (1-1.5 em) H4: 14-20 pixels (0.875-1.25 em) H5: 12-16 pixels (0.75-1 em) H6: 10-14 pixels (0.625-0.875 em) Adjust these sizes based on your specific design needs, readability, and importance of hierarchy.