lengths of posts such as 5, 5.5, 6, and 6.5 foot posts are great to use with standard livestock like cattle, goats, pigs, etc.
Shorter lengths such as 4 and 4.5 foot posts can also be used in rotational and/or mob grazing.
 
Reccommended spacing between posts depends on desired aesthetics, terrain, and contour. On Flat ground posts can be spaced as far apart as 75'.