Lauralee-
Sheets of concrete reinforcement mesh are something I've never seen-maybe a newer form of the rolls? The stuff we have(my late husband made tomato plant supports and, with a lightweight chicken wire tied onto the outside, protective "covers"-against the rabbits!-for our row of Ponderosa pines when they were tiny, and I still use them to put over where I have a gopher trap placed, to keep the dogs away from it!)out of this stuff. It came in a roll, probably 4' high, with openings of a uniform 6" square. It definitely DOES "want" to maintain the roll shape, as Susanne noted(Susanne, you are lucky that stuff didn't seriously injure you-it can be WICKED to handle!!) Maybe that's why it (also?now?)comes in sheets...?? I do think that cutting it down might be a bigger, harder job than one might think...and as I said, in my experience, it is made of a somewhat brittle steel, which I fear wouldn't last well in a fencing situation(as, when something puts pressure on it, it will bend and stay bent, instead of "giving", then springing back.) It would likely also have the same major drawback that cattle panel(and to a slightly lesser degree, hog/sheep panel) has-that a miniature foal "can" get its head caught in the openings. If using cattle/hog panels in relatively small areas,and where foals are likely to be, I might try placing them upside down, AND mounting them 8-10" or so off the ground(harder to do, as the panels are HEAVY, and will sag without the support of resting on the ground, unless you use closely spaced support posts,and frequent, snug ties of panels to posts.)
Tip: Get the little specialized "pliers" made for closing hog rings! It surely makes the use of them easier! I still have some hog rings out in the barn, from some fencing project in the distant past. They do come in different sizes, and the "pliers" are surely helpful; it can be very difficult to "hold" the rings to close them with ordinary pliers. Hog rings are too small to use to fasten fencing to T posts, though-they work well to fasten wire to wire. If you're using T posts, get the heavier weight ones, at least for where panels will overlap(you could then use lighter weight ones for the "in-betweens".) Be sure to ask for the clips that are the right size for the T posts you get. We used the clips with no problem when we built some fencing out of cattle panels, but if you can't get them to work, use a medium weight galvanized wire-I wouldn't recommend using twine of any kind, as exposure to weather will do it in, and, you cannot snug it up as it should be! The nylon ties would work, but will also be subject to deterioration due to exposure to weather.
Also-IF you want to be able to move fencing, it is best to use some sort of panel-type fencing. Though it generally costs a bit more, unless you are very knowledgable about fence building(which is an art in itself!), you will likely do better/be happier with cattle panels or other type panel fencing(which still needs to be put up 'properly', as noted in other posts on this thread!) to hold up well. Field fence,V, or diamond, mesh, 'non-climb'(the 2"X 4", tied, 'horse'fencing'), and other kinds of wire fencing-barbless wire, etc.-even lightweight galv. wire for electric fencing- all need to be STRETCHED when put up. IF you just "stand it up/unroll it", and staple it(to wood), or wireclip it(to T posts),it will sag as soon as the first horse rubs on it, and soon be neither safe nor very attractive! Properly constructed(for your local soil conditions!)stretch panels are needed on all corners, and on the straight, if there are LONG straight stretches. Steel wire contracts in the cold, stretches in the heat, BTW-which is a factor in how much it should be stretched.
It is VERY true that no fencing material is completely safe; however, over time, wire fencing that is good and snug has proven to be safer than that which isn't....even with panels, those that are upright, well-supported and well-fastened to their supports are proveably safer for your horses than those which sag and droop, with minimal supporting uprights.
(You CAN get welded rod(what cattle/hog/sheep panels are)panels, with 1" x 2" openings. VERY heavy, and horrendously expensive--but, it would probably be the BEST for fencing for miniature horses!)