General size of double beds is 72” X 72”. Along with this consider the height of mattress which generally varies from 4” to 9”. So, while buying a sheet calculate the size of product by adding surface size of mattress/product + the height of the mattress/product + a margin of at least 5 inches on each of the two sides to tuck in, in case of bedsheet under the mattress. This way you will be able to choose the right product size.

Say for example- If your mattress size is 72″ X 72″, and the height/width of your mattress is 9″. Then the size of the bedsheet required is

72″ + 9″ + 9″ + 5″ + 5″ = 100″.

So, at least the size of the bedsheet should be equal to or greater than 100″ on both sides.  

Similarly, the size of the mattress may vary according to the bed size, so it should first be measured and then the right decision should be taken.