Introduction:
In distributed systems, processes communicate with each other by sending and receiving messages. These messages can vary in size depending on the information being transmitted. The size of the message can be fixed or variable, depending on the requirements and design of the system.

Fixed-size messages:
Some systems require messages to always have a fixed size. This can simplify the communication process as each message is guaranteed to have a consistent format and length. The receiving process knows exactly how much data to expect and can easily parse the message accordingly. Fixed-size messages are commonly used in systems where efficiency and speed are critical, such as real-time applications or high-performance computing.

Variable-size messages:
In other cases, messages may have a variable size. This allows for more flexibility in the information being transmitted. Variable-size messages are useful when the size of the data being sent can vary significantly. For example, in a file transfer protocol, the size of the files being transmitted can vary greatly. Using fixed-size messages would lead to inefficiencies, as messages would need to be padded or split to fit the fixed size.

Advantages of fixed-size messages:
- Simplifies the communication process as each message has a consistent format and length.
- Allows for efficient processing and parsing of messages.
- Well-suited for real-time applications or high-performance computing where speed and efficiency are critical.

Advantages of variable-size messages:
- Provides flexibility in transmitting data of varying sizes.
- Eliminates the need for padding or splitting messages to fit a fixed size.
- Well-suited for applications where the size of the data being transmitted can vary significantly, such as file transfers.

Conclusion:
In conclusion, messages sent by a process in a distributed system can be of fixed or variable size. The choice between fixed or variable-size messages depends on the requirements and design of the system. Fixed-size messages offer simplicity and efficiency, while variable-size messages provide flexibility and adaptability to varying data sizes.