Yesterday I was in XXX city. When I was on the bus I realised that XXX city is (or was?) really big.
Should I use is or was? My point is that XXX city was big yesterday and it is surely still in the same size now, but the action "realise" happened yesterday ...
There's no problem with a mix of tenses but you'll hear both.
I realised Sydney was really big.
I realised Sydney is really big.
I prefer "is".