They are two different tenses.
Sería is in the condicional tense. In this sentence, it refers to an action that would happen in the future with respect to a time of reference in the past. It is similar to the conditional tense with "would" in English:
- It was a colossal Empire [...]. Its fall would also be colossal.
Debía is pretérito imperfecto. It describes an action that was ongoing in the past.
- Its fall would also be colossal, and also prolonged, because it had to span a long period of time.
It seems a strange choice of tense to me, as I would have used the condicional also here: "debería abarcar un enorme periodo de tiempo", or even better "abarcaría un enorme periodo de tiempo". The reason is that this is either a future action from the past perspective, or a hypothetical action. The pretérito imperfecto makes more sense if the action (the fall) was already happening at the point of time we are talking about, or if it is a direct consequence of another statement:
- Los enemigos del Imperio sabían que la victoria no llegaría pronto. La grandeza del Imperio significaba que su caída debía abarcar un enorme período de tiempo.