Depends on what you plan on doing in the house later if anything. Originally I ran 1” pex to the house, between actually heating the house, domestic hot water and the snow melt the temperature differential (delta t) was huge. Regardless of water temp, it takes flow to get real high BTU’s out. If you never plan on doing more than heating your domestic hot water and heating the house then 1” pex should be fine. Remember also, the pex sold in North America as one inch is actually more like 7/8” ID, they can sell it as one inch as the OD is the same as one inch copper pipe.
Would also help to know the size of the house, a big enough HX in the furnace and heating your house water could pull it down pretty far as well. This isn’t as critical on a stainless boiler compared to a mild steel one, but should still be avoided if possible.