What measurements would one need to calculate radiator output HW output
and henceforth boiler output for a domestic premises.

TBH, there is little point calculating the boiler requirement nowadays.
Almost all modern domestic system boilers are modulating (variable) types of
around 24kW. These will do a fairly large and poorly insulated house. Even
if the requirement exceeded 24kW, you would still fit the 24kW boiler, but
would just improve the insulation to bring down the requirement. Obviously,
hotels or other large or commerical buildings may genuinely require more.
Also, if installing a combi boiler, you'd probably want more than 24kW.

Of course, if you need to calculate individual radiator sizes, then you
would need to do the calculations. There are programs available to do the
calculations. You need to know the various dimensions of the rooms and
openings. It isn't done on volumes, but on wall/floor/ceiling areas, the
construction of those areas and the temperatures on the other side. You need
to know the materials used (or the appropriate u-Values) and details of
insulation. You also need to decide on the outside temperature you expect it
to work in. People generally design to -3C now, although -1C used to be
common.

When choosing radiator sizes, don't forget to derate to the average of
flow/return temperatures. Preferably design to 60C (or under) to allow even
greater efficiency for a condensing boiler installation. Even if you don't
install one this time (you should) then you'll be forced to next time, so
you might as well optimise radiator sizes for them. However, even if you
don't optimise radiators for condensing boilers, you'll still get much
better efficiency than with a non-condensing type.

Christian.