A lot of factors can affect how the heat can be transferred from inside a sealed enclosure to the environment. This include the environmental temperature of operation, the conductivity of the box, the airflow outside the box etc and really there is no simple rule of thumb to say how big a box should be used. Probably the bigger the better.
If you can use the PLC at lower operating voltage (say 12V DC) then heat dissipation will not be a problem and you can use as small a box as can contain the entire PLC without touching any conductor.
If you are running at 24VDC then you have to prevent the internal temperature of the box from exceeding the maximum temperature. The alumnium box is good heat conductor but the internal volume of the box should allow heat convection and there should be adequate airflow outside the aluminium box.
