Generally the bed sheet size specified should match up to your mattress size. If you have a queen bed, for example, you should opt for queen size bed sheets. However, if you have a deep mattress and the sheets you want aren't available in a deep depth size, sizing up will allow for extra depth. This is okay for a flat sheet as you can fold the corners and tuck in for a flat surface, but bare in mind, for a fitted sheet, it's important to get the correct sizing.