If the house is reasonably air tight, with at least code-min double-panes (or reasonably tight older windows + triple-track storms) it's unlikely that your 2000' farmhouse has a heat load of 75K. If it IS that high due to air leakage or degraded batts, it's likely that it can be brought under 50K, maybe even under 40K cost-effectively with some air sealing and spot-insulation upgrades.
125-140K is and INSANE value for 2000 square feet of living space- even a reasonably tight tent could come in under that at 0F, and your 99% outside design temp is a few degrees warmer than that.
It's worth air-sealing and insulating the foundation with 2" of closed cell spray foam, and putting down a ground vapor retarder on the floor. If you plan to continue to heat with oil (sounds like you do), it's also worth putting down at least R5-R7.5 of rigid foam on the floor and pouring at least a rat-slab over it. (2" of non-structural concrete is enough), but basement floor insulation is not nearly as critical as the air-sealing and insulating the foundation to at least a couple feet below grade.
On clapboard sided building it's possible to blow cellulose in over existing batts on exterior walls, which will fill in the gaps and tighten up the place considerably. Stud spacing & depths in 1835 literally NEVER conform to 20th century batt sizes, so it's highly unlikely the R11 batts are performing better than R8-R9 as-installed. As long as there is at least some plank-sheathing under the clapboards a cellulose blow from the exterior is pretty low-risk/high-reward on oil use. Cellulose installers can either pop a few clapboards and drill the sheathing to execute the install or drill through it all & pound in a wooden plug to be trimmed flush with the siding prior to painting. Cosmetically the former looks better, but it's less labor to just drill & fill.
If you have a zip code and a mid or late winter oil bill with a K-factor stamped on it, it's pretty easy to calculate an upper bound on the design condition heat load based on the DOE efficiency on the old beast.
The length of the existing baseboard has no bearing on the actual heat load, but having excess limits short-cycling on the boiler. Having "too much" isn't really possible, but it affects the near-boiler plumbing, since the return water coming back needs to stay above 140F where it enters the boiler. There are various methods for achieving this.
The heat load of the additional 500 square feet is within your control. Insulating the walls with either blown cellulose or spray foam insulation is much tighter than batts, and going with better-than code new windows (or low-E storms over antique windows) makes a difference. And insulating/air-sealing the foundation is far more effective than trying to air-seal and insulate the floor. It won't break the bank to get the heat load of that addition down to under 20BTU/square foot (10KBTU), or even under 15BTU/ft (7.5K), given that it has at least one wall in common with the rest of the house.
Bottom line, don't go any bigger than the smallest boiler in the lineup- at $4+ oil you can't afford the inefficiency incurred by even further oversizing, but you CAN afford to fix all of the lower-hanging fruit on the heat load. Air-sealing is the most-critical, followed by insulation. Not all air-leakage counts the same- sealing the foundation and the upper-floor-ceiling/attic-floor interface are the most-critical, along with any plumbing & electrical chases or flues/chimney-chases that extend from basement to attic, since that quells stack-effect leakage that runs 24/365, and runs ever-faster with decreasing outdoor temps. Balloon framed walls can act as stack-effect flues too, so it's important to have some air-retardency to the cavity insulation. R11 batts too low-density- little more than an air-filter in the wall cavity from an infiltration point of view, but putting even low-density cellulose in the cavities reduces that by 90% or more.