The corral panels nominally come in 10, 12, and 16ft. As a free-standing barrier, that is with no additional support, they depend on being set at an angle to the adjacent panel for rigidity. In a straight line, you will want at least a t-post every second or third panel. If set in a circle, ie a "Round Pen", you can certainly make a free-standing 60ft diameter pen; possibly a little larger, but the closer the cord sections (the panels) come to a straight line, the more unstable it becomes.
Horses are large strong critters, and if they take a mind to, they will fold, spindle, and mutilate even the stoutest portable panels (and themselves in the process), but for the most part they will not challenge them, and so enclosures from them are generally quite secure.
They are expensive; a 12ft panel averages probably $100 new. If you can find decent used ones, maybe $60 for the same panel. They are very useful things, both for semi-permanent and temporary enclosures. Spring for at least one walk-thru gate; ~$250.
All that said, you can build a smooth-wire-on-t-post fence for a fraction of the cost, and it will do just as good of a job keeping horses inside the fence line. Wire fences go up fast, with little effort, particularly if you can find a burly youth to pound in the t-posts for you.
Have fun with your new kids.
Edit to add: If you don't mind spending the money, just do your property perimeter in steel pipe fence and be done with it. This will certainly enhance the property value (as a "horse property") at least the cost of having the fence installed. For 40 acres you are probably looking at what? $100K? Anybody have a better guess?
Steve Jernigan KG0MB
University of Colorado
Last edited by george the mule; 11-26-2019 at 10:43 AM.