(Note: when talking about a 'gas', this article really means any free-floating tiny particle-like objects, usually particles or building blocks)
The Cosmic Cloud is a Multiversal natural object, caused by gases, or gaslike materials leaking out of Universes into their parent Multiverse. This is in the majority of cases completely harmless to other universes and they are impervious to the gas, either due to having a strong impenetrable surface, or because the gas isn't compatible with the laws of the other universe.
Cosmic Clouds don't have one single size, as Cosmic Clouds can be anywhere from a single quasi-gas or gas particle that escaped, to filling an entire Multiverse. In most cases, the Cosmic Cloud is a small chunk of the Multiverse, anywhere from 3% to 8% is the 99% of all cases of this phenomena.
However, most Multiverses simply don't have these Cosmic Clouds (like ours). Simply getting particles themselves to escape a universe would be considered a miracle for the majority of Universes.