its too variable man, you'll just have to test and see.. if your furnace pulls 5A and its so cold it runs continuously, you'd need ~230AH of battery every 24h.. if it cycles on/off and only ends up running for 6h a day you need ~90AH of battery.. if your furnace is variable output it can change how much power it uses and that changes it even more..
In winter daytime is shorter so solar charging gets harder, also stuff like snow will knock it out entirely if you cant clear it.. more than likely you'll want a genset to recharge fully in day so your batteries can run all night long without loosing heat.
Get something like a Victron Battery Monitor that shows you accurate consumption and runtime estimates, and figure out if you need more battery or not.. I know alot of the big furnaces like the Surburban ones can hardly make it through a cold night without a massive battery setup, I've woken up some mornings and literally every RV in the park had a dead battery and no heat, if it gets dangerously cold the'll often let those guys run their gensets all night long.