Yoda was about 900 at his death in Return of the Jedi, and if you go back 900 years from the 2010's, you land in the 1100's. Nero lived in the first century AD (the "000's" I suppose you could call it), so you'd have to live more than twice as long as Yoda did to see Rome during Nero's reign.
Perhaps we could decide that Duo is a Time Lord, in which case he could easily have seen Rome at that time regardless of age.
Big... what is big? Large, populated, great? By population of metropolitan area: 20 millions is huge, 10 millions is very big (megatown), 5 millions is big, 3 millions is quite big (not so big), 1 million is not big, 500 thousands is small, 100 thousands is very small. So, Rome and Barcelona are both quite big considering metropolitan population. But what about if you consider the city only? So any answer could be good ahahaha
The sentence depends on expectations. Rome could be not so big, but it is surely great. Lol