'Pre-firearms' would appear to be a rather difficult periodization to make. There is evidence of gunpowder weaponry such as handgonnes being used as early as the late 14th century. But, assuming you mean the 16th century with the so-called 'military revolution,' armies before that time and after Rome (which presents another problem, but that's another story) could be quite large. Charlemagne was able to raise multiple armies numbering at least 40 000 in total for his campaigns of conquest, and the total number of his forces could easily have topped out at 100 000. The armies involved in the crusades, in particular the first crusade, numbered in the tens of thousands, with at least 70-80 000 answering Pope Urban's call for the First Crusade. Edward I invaded Scotland in 1298 with almost 30 000, and Edward III fought the French with a similar number. Ferdinand of Aragon could amass 30-50 000 men to conquer Granada, and combined with Isabella's Castillan forces it was probably greater. I think this also suggests that, contrary to your question as to why there were so few or no 'great conquerors' post Rome, that there were indeed conquerors after Rome. Charlemagne, Frederick Barbarossa, Otto the Great and Henry I of the Holy Roman Empire, Charles the Bold, Duke of Burgundy, Edward III, Robert Guiscard, Philip Augustus, were all great conquerors and commanders. I've left out eastern Europe and the Middle East but Basil 'The Bulgar-Slayer', Heraclius, and many leaders of the Muslim Caliphates were no pushovers either!
That none of these figures were able to raise armies capable of equalling Rome is a result of the fact that, unlike Rome, they couldn't exactly marshal all the resources of the Mediterranean at their disposal. That said, a centralized or fairly centralized government/leadership, with the aid of bureaucrats and other nobles could raise large armies in the Middle Ages, particularly if their population and economy was sufficient to support one.
Sources: