Can I use an extension cord with this jump starter, and what size should it be?

Yes, you can use an extension cord, but you must ensure the pins match the charger's pins and the cord is properly wired and in good electrical condition. For a 120V charger with 0-25 feet of cord, use 18 AWG; for 26-50 feet use 16 AWG; for 51-100 feet use 16 AWG; and for 101-150 feet use 14 AWG. Always pull the extension cord by the plug rather than the cord when disconnecting.