I was looking online for the width of the thermostat at the wall, but could not find this explicitly referenced anywhere. Well now that I have a Nest Thermostat I can measure it; 2-5/8 inch, which is disappointingly narrow for the purposes of having the same footprint as a standard round Honeywell thermostat (about 3-inches).
Besides this one issue, so far so good.
I did have a bit of a wiring issue where I needed to connect the Red wire to the W-terminal and the White wire to the R-terminal on the Nest Thermostat for normal operation (which is purely the result of how the original thermostat was wired on the boiler). Apparently the Nest Thermostat wants the R-terminal connected to one side of the 24Vac transformer (the C-Terminal connects to the other side of the same transformer), with the W-terminal being connected to the thermostat lead from the boiler (heat-only thermostat, convective baseboard)
