Decks, Patios, Porches and Docks - pergola over deck

I am planning to build a pergola over an existing deck. The pergola will attach to the house (i.e., not freestanding) with these dimensions: about 8' deep (distance to house), 8'6" high, and about 17' wide. My plan is to have 3 posts, with about 8'6" between each. My questions: with these dimensions, is post size (4x4 or 6x6) a matter of aesthetics only? And if so, is one look or the other preferred in the industry? Also, since access underneath this deck is impossible, how should I attach the posts to the deck itself--toe-nailing (or screwing), or a metal connector (1/2 base perhaps?).....Any guidance on these questions and any other relevant issues would be greatly appreciated. Thanks!!

4x4 is acceptable for your posts.

Attaching directly to the deck is NOT recommended, but may be your only recourse. Look into other options.

I built a real nice free-standing pergola last year.