Start by measuring the dimensions of your silage pile, based on the style of silo you're using. Pile silos, for example, should be measured front to back as well as across, from the ground on one side, across the peak of the pile to the ground on the other side. Add a about four feet to each dimension to as a buffer, Your result will indicate the dimensions if you're ordering a single panel cover. If you'll be working with pieces, multiply the two final measurements. Your result will indicate the total square footage of cover you need to order.
If you're planning to use overlapping panels, you'll need to add an additional allowance for the overlap on each panel. Talk to your supplier about how your panels should be laid out and how much overlap is necessary for the type of cover material you're ordering.