Deck posts should be cut about 10 inches longer than the railing. The two most common sizes used for deck posts are 4×4 and 6×6. When using 4×4 posts, place them no more than 6 feet apart, and when using 6×6 posts, no more than 8 feet apart.

You will need to place your posts in the second half of the deck (from the midpoint to the cantilevers). Consider that the maximum spacing for 4X4 deck posts should be no further than 6 feet apart. Then 6X6 deck posts should have a spacing of 8 feet.