Half inch PEX would be more than enough to deliver 20 BTU/hr per square foot (more than you should ever need with a 2x6/R20, U0.30 window type house with R49 in the attic.)
Has anybody done the room by room heat load calculations yet? That's important to get it right, and to know just how much heat the floor/other needs to deliver. A typical 2100' ranch house over a 2100' insulated to code min basement with 8 or fewer corners to the footprint would have a total heat load of about 25,000 BTU/hr @ +10F (a typical
99% outside design temp for RI locations) or a bit less. An 900 square foot over 1200' 2 story with a 1200 square foot code-min basement would run closer to 20,000 BTU/hr @ 10F. The most you would
ever need to cover the Polar Vortex disturbance cold snap coolth is 1.4x the 99% load so assuming my WAG heat loads are right (could be higher or lower, depending on the particulars) call it 28-35,000 BTU/hr, which is ~10 kw of electric boiler, not more.
The cooling loads for houses that size will normally come in around 1.5- 2 tons in our area (I'm ~35 miles north of Providence). A ducted 2 ton Fujitsu
AOU/ARU 24RLGX puts out a bit more than 23,000 BTU/hr @ +10F, the 2.5 ton
AOU/ARU30RLGX puts out about 29,000 BTU @ 10F. They come bigger in that series (in half ton increments), but unless you're building the crummiest 2100'new house in RI you won't need anything bigger than
the 3 tonner, but even that's a stretch. Even the 3 tonner can drop back to as low as ~10,000 BTU/hr @ 82F in cooling mode, or ~9000 BTU/hr @ 47F in heating mode, and even if that's sub-optimally oversized it won't be a terrible fit for cooling or heating, but the 2 ton would be better if the numbers work, since it has a wider modulation range with a lower minimum modulation, which means it will idle along running nearly continuously for maximum quiet comfort. Some of the 2 and
3 ton Carrier Infinity w/GreenSpeed have less modulation range, others more depending on the air handler selection, but those too are VERY comfortable and quiet if sized correctly. Any of these would deliver about 3x the amount of heat per kwh compared to an electric boiler as a seasonal average, and even during cold snaps at temps cooler than 10F would be delivering 2x the efficiency.
For reference, I live in a sub-code 2x4 framed antique 2400' 1.5 story bungalow (400' upstairs, 2000' on the first floor) over 1600' of insulated basement (with a very inefficient 14 corners to the foot-print) a bit north of you in Worcester. Most of the windows are circa 1923 single pane wood sash double-hungs with 1980s vintage clear glass storm windows. My design load (at +5F) is about 35,000 BTU/hr, and would still be a less than 40K if fully heating the basement. Your house has to be substantially better than that, if any attention at all is paid to the energy efficiency of the building envelope.
How big is the big tub? (A 60 gallon soaker tub, or is it a 120 gallon spa?)