Most people recommend Django over Flask for larger projects. Personally, I've used Flask in bigger applications as well, and its a trade off -- Django saves you time with its app system, bundled ORM and MVC architecture, and the omnipotent admin interface, so you spend very little time reinventing the wheel. On the other hand, my raw speed of development is better with Flask (and I prefer Flask's debugging tools as well).
That being said, for a 'big' project I'd recommend Django over Flask, if for no other reason than googleability -- if you run into a particular error or question, Django's community (due to size) is probably going to be more helpful than Flask's.
That being said, for a 'big' project I'd recommend Django over Flask, if for no other reason than googleability -- if you run into a particular error or question, Django's community (due to size) is probably going to be more helpful than Flask's.