The building outside is quite big but when you go inside you see how massive it is. The main hall is very high ceiling, there are small souvenir shops (the cheapest postcards in Vietnam), it's pleasant to sit inside on a hot day as it's cool and windy in the hall.
It's one of my favorite churches in the world! When the sun shines through the glass, it's glorious. There is a great audio guide that explains each beautiful stained glass panel. Get your tickets online ahead of time to avoid the queue.