mix valve sizing question

slowflow

trying to size a mix valve for 26 unit house.

stick with the 1-1/4" size or go bigger? 

how is the watts n-170-m3?

recirculation pump necessary?



some units have 1 bedroom and some 2 bedrooms) but each unit contains  1 kitchen, and 1 bathroom (with faucet and shower).

pipes coming in and out of the tankless coil are 2".

cold water supply is 1-1/2"

return pipes are 1"

existing mix valve is 1-1/4" (old taco model # 587-1).

federal steam boiler, #2 oil, hot water comes from the tankless coil

need enough hot water for about 100  people (give or take, plus some have portable washing machines). 



stick with the 1-1/4" size or go bigger? 

how is the watts n-170-m3?

recirculation pump necessary?



Thank You In Advance.