Just on that point, I'm not sure how informed it actually was by convention (given it was the size of a consular army), but many Roman commentators fixed on 25-30,000 as being the "ideal" size for an army given that balance of effectiveness and ease of supply. More than that was "too big" and as konny says something done out of exceptional circumstances, like defense of Italy or civil war (and even then there were still upper limits).