what is the largest size string can be used .

Hi, there! Thanks for your question. For this unit #58620, use a 4 mm (0.155 inch diameter) mower line, with a length of 55 cm (21-1/2 inches). Do not allow the line lengths to vary more than 2.5 cm (1 inch). Otherwise, the mower head may not be balanced and vibrate during operation. You can view more information on page 18 of the manual at: https://www.toro.com/getpub/216578. Thanks!