"How do you figure out scale for making furniture or roomboxes?" MY TIME HAS COME!!!!!
I'm the jerk who keeps lecturing you guys on scale because you don't understand it. So scale consists of 2 numbers in a ratio: for instance 1/3, 1/4 and 1/6 are common scales for BJDs. That means that every 1 unit of measurement on the scaled object is equal to x units of measurement on the real object, where x is the second number in the ratio. For instance, for a 1/3 doll, every 1 inch on the doll is 3 inches in real life. So, if you want to make, say, a couch for a 1/3 scale doll, you would measure the dimensions of a real couch and divide each dimension by 3 to get the scale measurement.
Unfortunately, you have to know what scale your doll ACTUALLY is, and not what scale BJD people CLAIM it is, because BJD people absolutely refuse to understand the standard definition of scale. A 70 cm adult doll is 1/3 scale. A 45 cm adult doll is 1/4 scale. A 30 cm adult doll is 1/6 scale. A 30 cm kid doll is NOT 1/6 scale, though, because a kid is not the same height as an adult. If you multiply 30 cms by 3, you get the height of a real kid. If you multiply 30 cms by 6, you get the height of a real adult, or the height of a giant abomination monster baby.
So, how do you find out what scale your BJD actually is? You can do some math, or you can ask someone from the wider doll and miniature community, because we actually have to understand the standard, real, factual definition of scale to do our thing.