What does a tonne of CO2 look like?

Well, good question. Here's a photo of a grounded balloon outside Bella. A bit hard to judge scale, but it's rather like a hot air balloon.
Another way of thinking about it is a "standard(?) three bedroom house" - probably only in Oz - maybe in 1955; or as two double decker London buses, side by side. (Certainly different from the multiple black balloon approach of Aussie TV adverts...)