(Um, Yeah, you may notice that I had to use an assortment - the kids ate too many mini's and I had to add some jumbos. It worked just fine.)
Melt 1 lb mini marshmallows, drizzled with 2 T. of water, in microwave for 30 seconds, stir, melt some more. 30 seconds at a time, stirring between, until they are completely melted. (About 2 1/2 minutes).
Pour onto well greased (with Crisco) counter.
Knead the sticky mess (watch out, obviously the marshmallows are hot!) with hands well coated in Crisco.)
When most of the powdered sugar is worked in, add the rest of the bag. If it's dry (and it probably is, slowly add another 2 -3 T. of water, by teaspoons, working it in as you go) until all of the powdered sugar is absorbed and the ball is firm.
Continue to knead the dough (add a tiny bit of water if there are little clots of dry powdered sugar in your dough - I picked them out as they came up anyway). 8 to 15 minutes - til smooth and elastic.
Cover well with Crisco. Wrap tightly in plastic wrap, store in Ziploc with as much air as possible removed, and store overnight. (You can roll it out now if you must, as long as there aren't any little hard dry bits in it.) May be stored in the refrigerator for weeks.
When ready to use, bring to room temperature, knead until workable, and roll to about 1/8 to 1/4 inch thick.
Use on filled cake, frosted smoothly with 1/4" buttercream frosting.