Do you need a mattress of different sizes?
If you sleep on a side, the spine needs to be strait, so the level of firmness must be intermediate-low. If you sleep face up, you need more support in the back of the neck and in the lumbar area, so it needs to be firmest.
People with lumbar or back pains, must use mattress with intermediate firmness; about 5,6 degrees on the average European scale. People suffering from hiatus hernia also need an adjustable sprung bed base.
For people allergic to acarus, the best are natural mattresses with hygienic treatments that allow transpiration and avoid humidity.