Sizing the radiation may be a way to size a steam boiler, but a useless way to size mod-con. You size a mod-con by the actual heat load of the house, and adjust the outdoor reset curve of the mod-con to be able to deliver the heat with the lowest temperature at which it will keep up, using outdoor temp as the model of the heat load.
The lower the boiler's output temp, the lower the return-water temp==more condensing efficiency. Ideally you'd be need at most 140F water even at design conditions (and I'm guessing you are probably already there, unless you live in an uninsulated house and leave the windows open, with that much radiation.) If the return water is over 130F it won't be condensing at all. But even if you needed 180F water at 6AM on the coldest day of the winter, most of the time you wouldn't need water anywhere near that temperature for the radiators to keep up.
At +65F outdoor temps most houses have pretty close to zero BTU/hour heat load but at +25F most will have a fairly substantial heat load, but at +45F outdoor temps the BTU/hour requirements will be about half what it is at +25F. The BTU output of the radiators also vary fairly linearly with water temperature, so if you set up the boiler to deliver water hot enough to keep up with the load at 25F, and a lower temp water designed to keep up at a higher outdoor temp, letting the boiler's controls interpolate that line to match a water temp to an outdoor temp works pretty well. It's crude, but effective.
And as long as the mod-con boiler's high fire BTU/hour can keep up with the BTU/hour requirements of your house at the ACCA 99% outside design temperature for your area, it's big enough that you'd never get cold. But the SMALLER the minimum-modulation of the boiler is, the fewer burn cycles there will be, and the closer to the boiler's best-efficiency it will be at any given water temperature.
It's possible to put an upper bound on your boiler sizing by using the name-plate efficiency and the fuel use of the old boiler per base-65F heating degree day, which will also be pretty linear. If your oil vendor stamps a "K-factor" on the billing, that's the same information in a different form.