In order to ensure you have enough icing rolled out you can use a handy trick. Get a piece of ribbon and place it on the end of your cake board (before using buttercream or jam etc.) then place the ribbon across the edge then up the side of the cake over the cake, down the other side of the cake and to the edge of the board as per picture 1.
When stretched out this will have given you a rough idea of how wide the icing/fondant needs to be in order to cover your cake.
Now see illustration 2, if you roll your icing out to the length of the ribbon both lengthways and widthways it will ensure you have enough to cover the cake.
1. Place your ribbon across the board and cake.
Stretch out the piece of ribbon.
2. Roll out your icing/marzipan to the length of your ribbon, this will ensure you have rolled out enough to cover your cake.
It works for both round and square cakes.