Dave, page 166 has suggestions on the door, it comes with a recommended sliding door, this would place little force on the small studs. I would just go with the 6x6 posts then in the future anything goes.

It is a fairly short building and the top plate is used as the header. In different pictures there is a man door with a header as well.

The shed is a wonderful design, and us being human, there is no such thing as perfect, I find every building has a slight personal twist which necessitates change, accommodating our persona.

Tim