Why was the size of the Roman empire a problem

Bureaucracy! That is the main problem of the empire and mostly the republic. The republic fell because the size of the empire was to big, the events were moving rapidly and there was no time for bureaucracy, voting, debating and so on. When Rome became an empire it didn't of course change immediately. Some parts of the republic stayed enact. One of them was administrative bureaucracy. Luckily for the empire, after the third century crises, Diocletian started the dominate where the senate lost almost all of its power thus providing the empire with a more effective style of government where one man or two men, later three or four gave direct orders without having to debate and vote. That gave the empire 200 years more to live, although because of its size and changeable political currents it was never to last as a permanent state.

The central problem of the vastness of the empire was because of the type of government which was bureaucracy.