In other terms, you can (within limits) make radiators as small or as large as you like--they just have to ALL be reasonably proportional to the heat loss of each space.
There are however some problems if you plan for large radiators with low temperature requirement:
1) heat loss calculations are based on an "average" LOW winter temperature in your area--consequently the system will rarely be loosing the "full" amount of the heat loss.
2) Many boilers do not "like" low temperatures and as manufacturers have wrung the last drop of efficiency out of traditional designs their sensitivity to low temperature has increased greatly.
3) Traditional boilers achieve their best efficiency at higher temperatures around 180°.
Consequently, a system with low temperature requirement teamed with a traditional boiler MUST have measures taken to protect the boiler from low temperature--there are a number of ways but all add complexity, cost and are generally counter-productive fuel-wise.
Condensing boilers are fairly new. They are INTENDED to operate at low temperature and achieve their best efficiency when supplying water BELOW 140° or so. In general, the lower the supply temperature the higher the boilers' efficiency.
Flue gasses hold water--LOTS of water--POUNDS of it. It is never "seen" in conventional boilers because the temperature of the flue gas is high enough to keep the water in the form of invisible vapor. When the temperature of the flue gas drops below the condensation point (dewpoint) the invisible vapor appears in the form of water, but not quite pure water. It is slightly acid, about like tomato juice. If the condensation occurs inside a conventional boiler (or its flue) these components corrode--sometimes very rapidly.
Most condensing boilers aren't any more efficient at heating water initially. They gain efficiency by recovering the heat contained in the condensate--LOTS of it as there is so much water and water holds so much heat. The process itself is simple, but the realization is much more difficult.
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