1988-1999 Chevy and GMC full size the door speakers are six and a half inch on the extended cab the pillar speakers are 4x6 in. Some Silverados also had a small tweeter inside the door.
It has 4x6" speakers.
All the speakers in a 1991 Chevy Silverado should be 4x6 if they are stock.
They are 6 1/2 inch in the front door panels. If its an extended cab the rears are 4 x 6 inch. Let me tell you from experience replacing the rear speakers in a 97 extended cab is a pain, but can be done. Be cafeful not to break the panel clips then your panels will vibrate when the speakers are turned up.
Generally, a stock silverado will have four speakers; two in the dashboard and two behind the seat.
6.5" round speakers.
Front Speaker 6.5 4 Front Tweeter 1 1 Rear Speaker 4X6 2.5
Stock speakers should be 4x6's