The 1991 Buick Regal's speaker sizes varied depending on the trim level and whether it had the optional premium sound system. There's no single answer. You'll need to check your car's specific specifications or physically measure the existing speakers to determine the correct size.
Generally, you'll find either 4" or 5.25" speakers in the front doors and likely 6x9" speakers in the rear deck. However, this isn't guaranteed. Again, checking your car directly is essential.
