The lengths of the neck and bridge pickups in the original Fender* J-basses are not equal. Fender made the pickup bobbins so that their magnetic poles aligned correctly with the strings in the bridge and neck positions. Therefore, the neck pickup is ~3-5/8" (~92mm) long which is slightly shorter than the bridge pickup which is ~3-3/4" (~95mm) long.
Some modern designs made by Fender and others use two bridge-length pickups. A few use two neck-length pickups. We call the bridge-length pickups "Long" and use "-L" in our part names. We call the neck-length pickups "Short" and use "-S" in our part names.
Here is a picture and a drawing:
Things are a bit more complicated for 5-String J-Basses. Fender and other builders make 5-String instruments with different string spacing and therefore different pickup sizes.
The "Fender Standard V" 5-String J-Bass has 18mm string spacing and uses the same size pickups as the 4-String model. Most 5-String J-basses with 18mm spacing use this size.
The "Fender American Standard V" 5-String J-Bass has 19mm string spacing at the bridge and uses larger size pickups. In both cases, they have a shorter neck pickup and a longer bridge pickup. For the American Standard V (sometimes called "AS") the bridge pickup is ~4-1/8" (~105mm) and the neck pickup is ~4" (~102mm).
The Fender Squier VM Jazz-5 uses the same "American Standard V" size also.