The first and all important thing to do is to determine the actual design condition heat load at your house. If you have a history of fuel use at this house it's possible to use that to put a firm upper bound on it. Use a realistic 99% outside design temperature too, not some historical-low, since oversizing the boiler cuts into efficiency pretty badly.
Got a zip code and an annual fuel use number, and the BTUs in & out on the existing boiler's nameplate?
The LAST thing you'd want to rely on for a heat load number is a crummy "35BTU/foot" type of heat load per square foot of conditioned space rule of thumb. That's commonly done even by heat pros, and it reliably oversizes, often by 2x or more. While it may not have much of a n upfront cost/efficiecy impact for hot air furnaces, it's a cost and efficiency disaster for boilers.
The other thing you want to avoid is doing a BTU/foot calc on the baseboard for sizing the boiler, which is also a common but very BAD practice, even amongst "pros" who should know better. While the length of the baseboard is an important factor in the system design, it has no bearing on the size of the boiler. If you have the number of zones and the feet of baseboard per zone, let's have 'em, but it won't affect the boiler sizing.
At the current price of propane the pay-back on going with a more expensive modulating-condensing boiler is pretty short. The up-charge for a mod-con relative to a right-sized atmospheric-drafted cast iron unit may be close to a couple grand, but it'll use 15-20% (maybe even 25%) less fuel. If you're burning 2000 gallons/year of $3/gallon propane and say a right-sized cast-iron boiler cuts that to 1500 gallons, a mod-con that cuts it to 1200 gallons is still "paying" $500-600/year or more, so you get back the upfront cost delta in 3-4 years.
If you don't already have air conditioning, (or even if you do) heating at least partially with a ductless air source heat pump (mini-split/multi-split) cost well under half, and maybe under a third per BTU as with a condensing propane boiler. Even if it craps-out and doesn't meet the full load at your outside design condition any part of the heat load that can be supported with a mini-split will be cheaper than heating with propane. (Some are rated for 100% of the nominal heat output at +5F, many will still be putting out at least a decent amount of heat at -20C/-4C. Mitsubishi H2i series spec 70% of the nominal heat rating at -25C/-13F). Payback on a high-efficiency (HSPF>8.5) 2-ton mini-split against propane or oil heating is usually under 5 years even in high-priced electricity areas, and can be under 3 in low priced electricity areas.