Start with a 9-ft. Post and alter as needed to meet the overall composition of the elevation of the home. Avoid poles that are less than 8 feet, which will make the porch feel too low, or taller than 10 feet, which will make the porch feel uncontained. Piers should be located so they reach the middle of the floor joists. The height of a porch post depends on how high you want the porch to come. Generally, the higher it is, the more spacious it feels. Posts can be as tall as you need them to be; just keep in mind that if you need extra headroom, you'll have to install additional posts. Posts on a porch should be sturdy enough to support whatever you put on it, but not so big that they're a pain to deal with. They should also be close together enough that you don't get gaps between them due to tree roots or some other reason. As for diameter, that all depends on your taste and what you plan to use the post for. A porch post does not need to be thick; 1× lumber is sufficient for most applications. However, if you want the post to look good and add to the character of your home, then choose a wood that fits with the rest of your house instead of using treated pine or fir.