Hi there,
So this type of project has been worked on before here. You can find some posts that describe a similar use case.
I can address what I know on your points. Others will chime in and give you there thoughts as well.
- Physical size is always the limiting factor. But normally you would base line you r power requirements, Know what your operating current profile is and sleep profile too. Then given a power consumption current draw for this unit , use the Battery Size calculator https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life to get the days vs. power used info and some recommended sizes.
- Primoni makes that for the PICO board, it’s a small pcb that connects to the bottom like a sandwich, You end up with a small button under the USB-C connector that is I/O connected too and a long press powers-off the device and a single press when off , powers it back on. I wanted to make one for the Xiao it would work on all of them. the Schematic is online also. just no time to do it. (seeed should have that)
- (see #2)
I had the link but can’t find it atm… - NO , that is a function of the Battery Management System on each Lipo. OVER/UNDER and temp shut-off.
Xiao only stops charging at level and has minimum level for power-up. - Due to the WIFi and BLE I rarely see less than a 450mah rated battery used. YMMV (really depends on your resulting power profile for the whole device.)
HTH
GL 
PJ 
be sure to check out @msfujino (power ninja) posts on the Xiao Family battery and power profiles for sleep and Deep sleep. 