Most porch columns are spaced between 7 to 8 feet apart, while their length is usually 8 feet, 9 feet, 10 feet, 11 feet, or 12 feet. Diameter may be as small as 8 feet and over 12 inches- but keep in mind that these are just standard sizes (with an average diameter of around 10 inches). The size you need depends on your porch depth. You’ll also need to factor in porch railings and what style of columns you need. If you need to order custom columns, it may take longer and cost you more, but most companies are fairly accustomed to such orders and give you a fair quote.