Long and narrow is better, as long as the width is wider than your push tractor.  We put up a 400 ton stack for a customer.  I believe it was about 50' wide, 100' long and about 4'-5' high.  Definitely cover it.  We cover ours then cover completely with tire rings = no spoilage on top.  At one point I thought the stone covered with ag lime would work.  Not so sure now.  If you try it, I think the lime layer should be thin then let it get rained on then dried out before you put another thin layer on.  We tried one time on a very small stack off to the side of the larger one.  It worked ok but we didn't layer the lime slowly and didn't have time for it to get rained on and dried out so it could harden.  Eventually the rocks came through the lime.