For the 1996-1999 Taurus-Sable, 205/65-15 is specified.
That doesn't mean another size wouldn't work, but you might find some retailers won't install them!
NOTE: Every Ford product will have a sticker on a door jamb / pillar, or on a door itself, that specifies the approved tire sizes and tire pressure for that specific vehicle.
The stickers appear to be in a variety of places - even within a particular year & model. Look in these places on your car to find it. P205/65R15 is the standard size for 1999 Ford Taurus, Mercury Sable Listed on the driver's door post.
Original Equipment Size: 205/65-15
Open the drivers door and on the end of the door ( or pillar ) there will be an information sticker that shows the original size tire that was on the vehicle from the factory
BIGGEST TIRES?!? AS IN A MONSTER TRUCK SIZE?!? ABSOLUTLELY NOT RECOMMENDED! Please refer to your owner's manual recommendation.
Open the drivers door and on the latch pillar will be an information sticker that shows the original size tires on the vehicle from the factory
The 2002 Ford Taurus has a 3.0 L base engine size.
The 2008 Ford Taurus has a 3.5 L base engine size.
The 2003 Ford Taurus has a 3.0 L base engine size.
The 2010 Ford Taurus has a 3.5 L base engine size.
The 2011 Ford Taurus has a 3.5 L base engine size.
The 2012 Ford Taurus has a 3.5 L base engine size.
The 2007 Ford Taurus has a 3.0 L base engine size.
The 2001 Ford Taurus has a 3.0 L base engine size.
The 2009 Ford Taurus has a 3.5 L base engine size.