The next size is the 12” to 24” boulders. These are the most common boulders used for constructing retaining walls. These rocks should be handled with powered equipment to avoid injury and for ease of placement, considering a 24” rock can weigh as much as 700 lbs. Typically a professional landscaper will use these together with some 24” to 36” boulders and a skid-steer loader to construct a retaining wall.