The battery size for a 1997 Ford F-150 depends on the engine and options. There wasn't a single standard size. You
must check your owner's manual or the sticker on the battery's top (if it's still there) for the correct Group Size.
Common group sizes for that year's F-150 include Group 65, Group 75, and sometimes Group 78, but this is not exhaustive. Getting the wrong size could lead to fitment problems or even damage.