That's consistent with the recommend, yeah. Though bigger means moving less frequently, since it will take longer for them to eat the goround bare.
Next question is how you plan to move your tractors. If you are using manpower, weight is an immediate restriction on size.
Your grounds are also a concern. If they are flat, you have no size constraints, even if not level. If your ground is rock, rolling, or rutted, multiple smaller tractors are superior to one large tractor that may not sit flush.
and my general prefence in design is, functionally, a rolling "hoop coop" made with cattle panel and a sturdy wooden frame. Its resource efficient, cuts weight, and (properly constructed) as sturdy or more than a "traditional" boxy build.