Sizing heat loss from a building
The basic principles
The idea behind sizing is twofold, firstly to establish the heat requirement of each room and then (by adding all the room requirements together) to establish the total heat requirement for the property  this page is written around a domestic resistance but the principles can be adopted for any sort of building.
Thermal energy moves from a temperature to a lower one, the bigger the difference, the faster the transfer. As well as the temperature difference, the material through which the energy moves also affects the speed of thermal transfer. Each type of material has its own particular rate of thermal conductivity, this is usually referred to as it's 'kvalue' and is quoted in Watts per square metre/degree Celsius difference for a one metre thickness of material (or in imperial as BTU per square foot/degree Fahrenheit for 1 inch thickness).
For different building materials/methods, the thermal conductivity is quoted as U values, these are calculated from the kvalue of the materials used taking into account the thickness of each material and material combinations. U values are quoted as in metric or for imperial. This makes life easy as a single figure is available for calculating thermal loss through common construction methods.
From the above, it can be seen that to calculate thermal loss from a room, it is necessary to establish:
 The temperature difference across each wall, ceiling and floor (this is done by defining the required internal temperature and the anticipated outside temperature).
 The type of construction of the walls, ceiling and floor, from this the appropriate U values can be established.
 The area of each material.
There is one more factor to take account of, namely Air Changes. No matter how well a room is draught proofed, there will be a certain amount of natural air change  if there were not, people would tend not to live long as all the oxygen would be taken out of the air! Air changes are quoted as so many changes per hour (i.e. the complete volume of air in the room changes so many times).
The figures:
Room temperatures  °C  °F 
Lounge  21  70 
Dining Room  21  70 
Bedsitting Room  21  70 
Bedroom  18  65 
Hall and Landing  16  60 
Bathroom  22  72 
Kitchen  18  65 
WC  18  65 
Temperature difference
The temperature difference should not be thought of as just between the inside of a property and the outside temperature; the differences between adjacent rooms should also be considered. In the UK, the recognise temperatures for domestic rooms are given in the table to the right.
The temperature outside of the property is normally (in the UK) taken as 30° F (1°C) which is supposed to represent the normal lowest winter temperature.
U values
Typical U values for some of the more common types of construction are given in the chart included on this site.
Area of material
The area of each type of material used, is simply measured. Where a window or door is included on a wall, just measure area of the whole wall, and take away the area of the window/door. For metric calculations, measure the areas in square metres whilst for imperial use square feet.
air changes per hour 

Lounge  1 
Dining Room  2 
Bedsitting Room  1 
Bedroom 
0.5 
Hall and Landing  1.5 
Bathroom  2 
Kitchen  2 
WC  1.5 
Air changes
Again in the UK there are recognised Air Changes for each room (see right). Where the rooms have not been draught proofed, the number of air changes should be increased.
With these air changes, it is necessary to calculate the energy required to heat the volume of air by the temperature difference. To heat one cubic metre of air by one degree Celsius required 0.36 watt (or for imperial 0.02 BTU to heat one cubic foot one degree Fahrenheit).
So the energy loss is simply:
the appropriate watt or BTU figure x volume of the room x number of air changes x temperature difference
It is normal to take the temperature difference as being the design temperature of the room and the outside temperature (i.e. in the UK 30° F (1°C)).
To be on the safe side, a universal figure of 3 air changes per hour can be used with confidence.
Aspects not included
The above calculations do no take into account thermal energy introduced within the property. The two main contributors are the people occupying the property and the heat generated by cooking and use of hot water for washing etc. In general these contributions can be ignored.
And finally, take care with your calculations
Remember that all calculations must be made in either metric or imperial units, don't mix square feet with degrees Celsius or watts with degrees Fahrenheit.