If the chickens are in the tractor all the time they generally need 4 square feet per chicken in the coop and 10 square feet per chicken in the run (you may be able to get away with less for bantams, but less is not really ideal).
It sounds like with your situation of having a small yard and it being a tractor and not free ranging that designs where the run is also beneath the coop may be good for you because your space is working double for you.
Unfortunately not all the articles spell out exactly how many chickens they are for, but you can look at dimensions or adjust the dimensions of the plans to better meet your needs. For example you could have a tractor where the run is 4'x10' with the coop the same 4' width x 4' long and that would suffice.
This one looks like a contender...
https://www.backyardchickens.com/articles/suburban-chicken-tractor.48116/
You can make any coop cute to appease the neighbors later, paint, a bit of contrast trim around windows, an attached hanging flower pot, a cute sign. I get this piece of of it for sure. My one neighbor cares and our yard was a mess during the construction phase with scrap wood and tools and stuff for like a month due to weather. Now I have ducks too, but the duck house is not attractive at all for the moment and won't be made "cute" until spring rolls around.
https://www.backyardchickens.com/articles/chicken-tractor.57131/
https://www.backyardchickens.com/articles/our-first-chicken-tractor.48924/
https://www.backyardchickens.com/articles/our-chicken-tractor.49045/
This last one is a lot like the design you were originally looking at but a bit bigger coop space and looks a bit sturdier.
https://www.backyardchickens.com/articles/green-chicken-tractor.63626/