We offer a free survey to establish this and advise on the heat output you require, as well as design options for your hearth and surround etc. The ‘size’ is broadly calculated using a standard sum which is the length x width x height of your room and then dividing the number by 14 – but speak to us for clear guidance because this is just a rough guide and every installation is different depending on things such as double glazing, age of the property etc.