As stated, same width as rears. Ideally you want the front tires and rear tires to track independently of each other to contact fresh packed dirt (although most orgs require a 1" overlap of centerline or similar).
Due to having a frame wide enough to accept a motor/tranny in the front and being able to steer (without having issues with the tires hitting the framerails), it's easier to narrow the rear tires (most run a narrow section of frame in the rear with the tires almost touching) than the front. Hence the wider stance in the front.