There is a dispute among historians over whether the Roman Empire collapsed because of its increasing size and system-imminent flaws which were not suited for this size. The bigger the territory, the harder to govern it efficiently. It is possible this factor could also be viewed in regards to today's large Russia and very large companies where the communication lag within a system increases.
This brought me to the question: How was communication in the Roman Empire set up, and did it exist in a holistic way at all? How were orders and information (feedback from marginal regions) transported or brought to important decision makers? Did the Romans use horsemen, runners, carrier pigeons, or a form of postal service? How reliable was this and on what time schedules did a person-to-person exchange happen (letter and reply letter)?
Can anyone here shed some light on this process and its limitations, or implications for a consistently increasing Roman Empire? Is it safe to assume it must have played an important role when analyzing the success and then the demise of the Roman Empire?