Several factors need to be considered to choose the correct slip on flange dimensions. Firstly, the flange must match the outside diameter of the pipe it connects to. The pressure rating must also be considered, as the flange must be rated for the maximum pressure of the system used. The material must also be compatible with the fluid or gas being transported. In addition, it's essential to ensure the flange's bolt pattern matches the system's other components. The flange must also be thick enough to handle the pressure and weight of the connecting parts. Lastly, the flange must have a smooth surface to prevent leaks and ensure a secure connection.