A round pen is an indispensable training tool for the horse enthusiast. It offers you a safe, confined area to work with your horse either on a lunge line or while riding. This wooden round pen can be built in a weekend and enjoyed for years.
A usual round pen is 60' in diameter, with 4" posts (4'-6' high) every 8'. Remember to factor in the 6' gate (4' for stalls). See https://www.homedepot.com/p/4-in-x-4-in-x-6-ft-2-Ground-Contact-Cedar-Tone-Pressure-Treated-Timber-159740/205971065 for cheap cedar posts that are 4in x 4in x 6ft and are pressure-treated.