What is the universe in thermodynamics and the size of universe?
The universe in thermodynamics is the combination of systems and surroundings. We know that the boundary of the system but here is the point to know a boundary of the surrounding.
The boundary of a system depends upon what we consider as a system, for example, hot water in a bowl, the bowl is a boundary of the system and the heat or mass that transferred from the bowl beyond the limit of the surrounding is the boundary of the system.
The size of the universe depends on the size of the system. If the system you considered is more heat then the heat or mass transfer area is increases then the size of the surrounding boundary also gets increased.
