With low-efficiency cast iron boilers it is generally accepted that any size up to 150% of design load is OK. The high mass of cast iron absorbs and holds heat somewhat mitigating the short-cycle penalties. Since you have a low efficiency appliance and natural draft the size factor is not a great.
If you are condensing steady state and shoulder season performance can be dramatically compromised if the low-fire exceeds the design load...not all that uncommon. The answer is for the homeowner to insist on a sample of a proper Manual 'J' heat load and his own once the deposit is made and before the unit is delivered.
They look like this: