Hi John
A 10 ton coil would be roughly 4 to 5ft wide or high, with a Depth of 4 to 5 ft. With the Coil Eye/Bore, could be anything from 1 & 1/2 feet to 2 feet wide. A 28 ton Coil would be roughly about 6 ft high with a depth of around 5 feet and an bore of around two and a half feet. Coils come in all different sizes. Plus you have to think about the thickness of the actual plate.
I did see an article in Model Railroader a few years ago, the Auther used an appropriately Coloured Sand Paper, cut into long strips and then wound around a pencil to form the Coil. Using PVA to hold it together. Then he used very thin tape to use as the Steel Banding. Then he weathered it
I hope this helps.
Simon