Navien Sizing. What size boiler do you currently have? The domestic hot water heater for the smaller.
All navien combi boilers are oversized for your heating load, but at the sometimes glacially cool incoming water temps you can't really downsize much and still have reasonable. The next smaller size is rated 80k btu for heating which should be adequate for you house and climate. Navien condensing boilers from the nhb series, with the maximum size of 24″ x 17″ x 13″ and with the lower power output, are the smallest units from this manufacturer.