I'm trying to understand something - why have it sized for EXACTLY what you need? You just mentioned expanding the home yourself, so it's not out of the question that it may happen for others down the road. There's also the issue of degradation over the years - more heat loss within the home.
We purchase many things every day and we never aim to purchase "just enough". If you have natural gas, it isn't expensive and neither is the electric it takes to run the system. Lastly, these boilers jump from 80,000 BTU to 100,000 BTU for the next level up so getting it exact may not even matter.
For the money you're spending on getting a company to measure you, you're losing any perceivable savings anyway. I realize people treat this like a science but in practical terms it provides no real benefit because of negligible cost differences.
The consumer guides to furnace selection say that an oversized furnace results in "short cycling". They say short cycling is bad for several reasons including but not limited to being uneconomical. In my case you are correct that the potential saving is small. The 25-year old furnace in my 915-square foot house is 94K BTU. My total gas consumption for the past eleven months is 632 therms at a cost of $448. This includes space-heating, water heating, gas oven and cooktop, and gas clothes dryer. This eleven month period had an above average number of heating degree-days for my location. I've been advised that a professional Manual J sizing would cost me $150-$300.
As Joe C mentions, short-cycling produces uncomfortable heating. An article I saw on the web mentions another drawback of short-cycling. To attain higher efficiency (AFUE), the heat-exchangers in modern furnaces are made of thinner metal than in older furnaces. Short cycling causes repeated thermal expansion and contraction which can eventually ruin these thinner heat exchangers via metal fatigue.
I just completed running loadcalc.net. The result was 22214 total BTU's heating. Thus, a 44K BTU 80% AFUE furnace would suffice. One of the two HVAC contractors who gave me estimates (and who recommended a 60K BTU furnace) told me that his computer program (which he didn't name) told him my house needed 24K BTU. (He took no detailed measurements of my house.) He said he would only install a 44K BTU furnace in a condo where there are adjacent units, not in a house with four walls.
Joe C: Thanks for your links to Slant-Fin and Taco Manual J software. I browsed one of the user manuals and viewed the Taco video. My PC's are Linux only (without the Wine MS-Windows emulator ) so I can't run either of those .exe files.
Joe C: In addition to the "envelope-tightening" measures you listed, I've read that duct sealing should also be done. Leaky ducts can lose up to 30% of heating energy. I'd like to have a duct leakage test performed but I expect I'll have a tough time finding someone to do it.