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
I guess like in English, "muy grande" is a relative term. I suppose the question is "compared to what?"
When they said "Inglaterra es una isla" I reported it as an error since it's factually incorrect. I know it's just a sentence to work, but that seemed a little blatant an error.
In terms of land area, population, and in particular historical importance Rome is one of the great cities of the world (and a place I will be visiting about a year from now), but if you went from New York to Rome, you might say "it's not very big."