What is the maximum length of the Atlantic Ocean (if the length may be traced in the middle of the widths all the way)?

Ignoring the difficulty of determining the midpoint as you move from north to south, there is a bigger issue in that the northern and southern endpoints of the Atlantic Ocean are not clearly defined - it depends on who you ask! As the image at the link shows, there are two different basic definitions for the southern boundary, one stopping at latitude 60S, one extending all the way to Antarctica. The two will give significantly different lengths! The official IHO definition, from 1953, goes to Antarctica; the termination at 60S was introduced in 2003, due to the proposed addition of the Southern Ocean, including all waters south of that point. This introduction reflects the way the waters behave, and is widely used, but has not been officially ratified.

https://en.wikipedia.org/wiki/Borders_of_the_oceans#Atlantic_Ocean

As the crow flies, the straight line north-south distance from Kangerlussuaq, Greenland to Coronation Island is 14,196 km (8,821 miles), using Atlantic boundaries at iilss.net.

https://www.freemaptools.com/how-far-is-it-between.htm
http://iilss.net/border-of-seas-and-oceans-in-the-earth

You can also play around with this site to generate a more accurate distance, using latitude and longitude:
https://www.gpsvisualizer.com/calculators#form:distance
I don't think there's a website which will allow you to calculate a distance which follows a non-straight route.

You might be able to use Google Maps to do this - there is a way of measuring distances by putting markers on wherever your route changes direction. I was using it on roads a couple of weeks back, and there you have to put a marker when you turn a corner. It might allow you to dot the mid-line of the Atlantic on a very small scale view. Won't be 100% accurate, but as there's no clear division of oceans (and some people dispute the existence of the Southern Ocean as well...) it'll be as good as any - and depending on your judgement of the middle positions might give quite a good result.. I did it by right clicking on the map and taking the 'measure distance' option. Just tried it in the sea and it does work.