The correct answer is option 'A', First Generation.

The computer size was very large in the First Generation of computers. Let's explore the characteristics of each generation of computers to understand why this is the correct answer:

First Generation:
- The first generation of computers emerged in the 1940s and lasted until the mid-1950s.
- These computers used vacuum tubes as their primary electronic component.
- Vacuum tubes were large, fragile, and generated a significant amount of heat, resulting in the need for large computer systems with extensive cooling systems.
- The computers of this generation were huge in size, occupying entire rooms or even buildings.
- They required sophisticated infrastructure and dedicated spaces to house them, such as special computer rooms.
- Examples of first-generation computers include the ENIAC (Electronic Numerical Integrator and Computer) and UNIVAC I (Universal Automatic Computer).

Second Generation:
- The second generation of computers emerged in the late 1950s and lasted until the mid-1960s.
- These computers used transistors as their primary electronic component, which were smaller, more reliable, and generated less heat compared to vacuum tubes.
- The introduction of transistors made computers smaller and more powerful than their first-generation counterparts.
- While the size of computers decreased compared to the first generation, they were still relatively large and required dedicated spaces for operation.

Third Generation:
- The third generation of computers emerged in the mid-1960s and lasted until the early 1970s.
- These computers used integrated circuits (ICs), which further reduced the size of electronic components.
- The use of ICs allowed computers to become even smaller and more powerful than second-generation computers.
- However, computers of this generation were still relatively large and required specialized rooms for operation.

Fourth Generation:
- The fourth generation of computers emerged in the late 1970s and lasted until the early 1990s.
- These computers used microprocessors, which combined all the functions of a central processing unit (CPU) onto a single chip.
- The use of microprocessors led to the development of personal computers (PCs), which were significantly smaller and more affordable than previous generations.
- PCs could fit on a desktop or even be portable, marking a significant decrease in size compared to earlier generations.

Fifth Generation:
- The fifth generation of computers emerged in the 1990s and continues to the present day.
- This generation is characterized by the development of artificial intelligence (AI) and advanced technologies like natural language processing.
- The size of computers in the fifth generation varies greatly, ranging from large supercomputers to small, portable devices like smartphones and tablets.

In summary, the computer size was very large in the First Generation of computers due to the use of vacuum tubes as their primary electronic component.