A method for setting the sizes of film tubes extruded by blown film plants includes changing the film tube size from an initial size to a final size, and optimizing the location of a frost area during the size change by setting the output of a cooling fan. Physical parameters of the initial and final size of the film tube are stored in a control device. The control device takes these physical parameters into account when generating control signals. The control signals form the basis for the control of the cooling fan output.