A printer control unit (3) is described for controlling the size of droplets emitted by a nozzle of a printhead (8) in an inkjet printer. The control unit (3) receives information about an image to be printed by the printer, for example from a raster image processor (5), the information including droplet size information regarding the size of droplet required to be emitted by the nozzle to print the image. Print parameter information for printing the image is determined on the basis of the received droplet size information, the print parameter information including information for use in setting the printer such that droplets emitted by the nozzle during the printing of the image have the required droplet size.