export from archicad to 3ds file,import it in blender (scale doesnt matter)
Then draw a cube (press space - add mesh - cube)
select that cube and press "n"
then adjust size to x, y, z, to be 1
Then you will see a cube sized 1 meter and you will be able to compare to your model
I had to scale UP my imported models 100 times (select your model and scale it up 100 times)
after that you can do texturing and export it to octane
units are meters !
I hope tha tyou will understand this quick step by step
if you have any problem, write here man and i will try to help