From the factory standard was 11/16" and heavy duty was 7/8" on both 6 cyl and V8. The 71-72 V8 came with 5/8" as standard.
The biggest sway bar doesn't necessarily make it the best. It depends on what you plan to do with your Maverick. I for instance have an Addco 7/8ths sway bar on my Comet that I use for street driving and drag racing. A 7/8ths sway bar is not the best choice for weight transfer when drag racing. I could loosen it at the track but I don't. That 7/8ths Addco bar sure helps with body roll on the street.
even though these cars are light overall.. the're also nose heavy and not known to be the best handling to begin with. I'd go with the PST bar mentioned above at the very least. The factory stuff is weak junk that makes you want more when it's really needed. if you plan on driving it harder with bigger stickier front meats?.. then maybe look at Addco's 1 1/8" front bar kit. That bar was created within the last couple years through a group buy deal and turned out to be a fiasco of Addco communication errors with 3 different bars arriving at my door step. Haven't installed mine yet, so I can't say for sure.. but AFAIK, all those who have installed the final iteration have had no fitment issues.
I have the heavy-duty 7/8" factory bar on the front of mine. It originally came with the 3/4". I also have polyurethane bushings on it. I had the Addco rear bar on it until I installed lowering blocks, which caused interference issued between the bar and the passenger-side shock. I made my own mounts though. The Addco mounts are horrible and can cause damage to the frame rails. Your question was regarding the biggest that will fit, which I would imagine is the custom 1-1/8" bar.
Largest: http://www.jegs.com/i/ADDCO/020/549...RfUyW25qnSnUQZ6a2AI8eW_JD1DgXSF9vMaAr0j8P8HAQ For mustang, I have the 1" bar, seen here: http://www.ebay.com/itm/1964-1966-M...by-/161903113332?_trksid=p2141725.m3641.l6368 Both by Addco, The 1" bar made a very nice difference around corners, both in maneuvers, and just how it felt in a regular driving situations around corners I remember reading something about the 1 1/8" bar having some sort of interference issue, but i can't remember where, or how it was dealt with... I had to move the mounting points forward as far as I could to get the end link geometry as straight as I could, but it did fit, just drilled another hole. You mentioned this is for street driving, and not autocrossing or drag racing, I highly recommend the 1" bar, as you can see many have the smaller 7/8" bar and are satisfied. If you want, you can install stiffer springs, and a rear bar at a later date, and this will help even more.
If you have stiff rear springs, you don't need a rear bar at all, although the ride may be unacceptably harsh.
