The reason that information about vertical posts is generally not included in code is that computation of the allowable load on a post is more complex than for a horizontal member. Typically, allowable vertical load is controlled by the compressive strength of the wood for SHORT posts, however once the post is beyond a critical length, the allowable load is controlled by buckling of the post. The exact length at which buckling begins to control is not simple to determine, that's why engineers go to school.
The length at which buckling becomes an issue is a function of the length of the post, dimensions of the post, end conditions (i.e. is the post buried in concrete or nailed to a beam), and whether there is any intermediate support of the post (i.e. cross bracing). This is typically beyond code, however I realize nobody wants to hire an engineer for a 2 foot deck off the back of the house, so the local building inspector can probably tell you what size you need for that. If you are building a high deck, like for example an 8 foot high deck to match the second floor of the house, you might need a structural engineer to size the posts.