Hey Dutch Master, there is an easy way to understand this.
It's NOT the size of the room that is a consideration.
It's the size of the PLANT CANOPY.
Plants grow as a result of Photosynthesis. There is a DIRECT relationship to the quantity of lumens striking the plant canopy and the amount of growth and what type of growth a plant will have.
You can use candles from 8 feet above your plants to veg with. You'll end up with skinny, long legged, unhealthy, wispy little half dead plants, but they'll grow some.
However, if you're looking for healthy, thick stemmed, short node, fast growing, thriving plants, you need to approximate what the Sun does in natural light.
The sun, at high noon on the equator, provides about 10,000 lumens per/square foot of surface.
It's not feasible to use 10K lumens in a grow room, because the costs would be astronomical, so 5,000 lumens per/square foot of PLANT CANOPY is an amount that most experienced growers accept as a standard to provide as good of growth as can be, inside.
When the plants are small, they have a much smaller "footprint". That is to say that the PLANT CANOPY is much smaller than it will be when the same plants are flowering.
If you have 12" plants or less, they can be placed closer together where the PLANT CANOPY is a much smaller area to light.
When this is done, it takes less light to make 5,000 lumens per/square foot of PLANT CANOPY than it will later when the plants take up much more area.
So, to make your plants grow at the best overall rate and with the best overall health, you should use 5,000 lumens per/square foot of PLANT CANOPY.
(I've emphasized PLANT CANOPY to make it clear to everyone that the room size IS NOT what you base your light levels on.)