is a basement an option? to save materials i just dig a basement under my longhouse in the meadows for the portals. it has 2 floors. so technically i have a 4 floor building. more than enough space for all the portals. and i only use corewood for structure. the stones are just walls i added later and i never use iron beams. nothing is ever broken.
https://steamcommunity.com/sharedfiles/filedetails/?id=2717864998

a 2nd building i build in the plains with stone, is smaller due to the limit. had to use 1x1 stones as a pillar to stabilise the floor. never tested the maximum limit how big u can build because i raise a small island where i build the fields for flax. but i think u have to use a pillar every 3 stones to keep the surface save.
https://steamcommunity.com/sharedfiles/filedetails/?id=2717402949

https://steamcommunity.com/sharedfiles/filedetails/?id=2717395811
in the harbor i build a bridge with stone and have to use pillars to stabilise. there i have 4 stones and then a pillar.

and for the bed a brazier works too. when u cant use a fire or have no space for it.
https://steamcommunity.com/sharedfiles/filedetails/?id=2718610551
bed over the throne and no fire.