Most any file with a PPI of 180 or above will print fine but the best size is one that divides evenly into the target printers resolution (DPI). For Example, if you are printing to an Epson 3800 at resolution of 1440 DPI, a good PPI size would be 360 PPI since it would be exactly ¼ of the print resolution (360 x 4 =1440) or 240 PPI since it would be 1/6 of the print resolution (240 x 6 = 1440). By using a file size that divides evenly into the print resolution you get no remainders which results in a more efficient process. This isn’t a large improvement but it is an improvement.