To select the right size of angle steel for a project, first, accurately calculate the expected loads, including static and dynamic loads. Consider the type of load (compression, tension, bending) and its direction. Then, determine the span and support conditions of the structure. According to engineering mechanics principles, choose an angle steel size that can meet the strength, stiffness, and stability requirements. For example, for a structure with heavy loads and long spans, larger - size angle steel with higher load - bearing capacity is needed. Refer to relevant design codes and standards, and consult with experienced engineers to ensure the safety and reliability of the project.