The amp ground should be very thick and very short. I needs not run all the way back to the battery. Any bare metal surface (ie. unpainted) that's near the amp will do fine.
I'd recommend trying to keep the amp's ground wire no more than 3 feet if possible.
Not sure what you mean by ground lead wire... might you be refering to the wire for the remote turn on lead? Remote turn on leads are very thin... like as thin as a headphone cord for like a walkman. Where as the ground power wire should be thicker like, thick as or thicker than a pencil.