When we bought our last but one (*) toaster, I took a cardboard template and was able to discount every single toaster in Currys except their cheapest own brand model which was allegedly a "4-slice" but (luckily) there were only 2 - longer - slots. This meant it could actually toast 2 slices.
*the best way to make toasters more efficient is to make them last slightly longer than a few months. The template experiment was 2012. Given our current toaster is less than 2 years old, it must have lasted about 2 years.