standard boilers always were (wrongly) oversized, but tradition always taught that whatever you do don't undersize. But temp diff. was always 11deg, condensing boilers rely on good temperature uptake in the primary heatexchanger to rob the latent heat from the flue gas, hence 20deg diff. recommended.
To give a working example, my house today has a standard oil boiler, ranged to 110,000btu and next month I am ripping it out to fit a grant vortex (best on the market IMHO) and will install 90,000btu to put it under pressure and ensure condensing takes place as close to 100% of run time as possible.
Lets say I just banged in 110,000btu condenser, the boiler will condense as the house is heated up (ie. return temperature is still cold) but once circuit is running at temperature (ie. 10 deg diff.) then the boiler will actually stop condensing and revert to
something very close to a standard boiler for the remainder of the heating time period!