If a radio receiver is placed too close to a radio transmitter, the receiver will saturate. Even if the two devices are on different channels, they will interfere with one anotherand if they are on the same channel, the power received will be too high for the radio to work properly. Some Bluetooth radios have a high sensitivity receivers and will not work reliably if they are put within 10cm of one another.
So, there are practical limits on how close some devices can be placed. Besides, if we had to get devices right on top of one another, we might as well join them up with cables. And if we had to do that to get them to work, we'd have lost all the convenience of Bluetooth wireless technology!
Let's assume that our piconets are spaced out enough that they won't interfere with each other if they're on different channels. Still, they're close enough together that they will interfere if they transmit on the same channel at the same time.
It doesn't really matter how big the room is, as long as it meets two qualifications: It's big enough to fit all our piconets in with the minimal spacing between devices to stop radios saturating, but it's small enough for piconets to interfere if they collide on a frequency.