While the dimensions of a bed frame should roughly match the mattress size, it will have a few extra inches added to the width and the length. This is done to make space for the bedding, so you can lift the mattress to put on the mattress cover and fitted sheet. The design of the bed frame will also be a factor in its size. Before choosing between a double or a queen mattress, factor in the size of the bed frame and add that to the overall amount of space you will need to fit it all comfortably into your room. If you thought a queen mattress would fit but you didn’t take into account the size of the bed frame, you may be sorely disappointed in the end result.