I have successfully grown very small balloon flowers from seed and wonder if I bring them in during the winter or leave them outside…,they are in a flower box with a good depth of earth
Balloon flowers (Platycodon grandiflorus) are perennials, i.e., they come back every year. They are hardy to Zone 3, so certainly can survive Toronto winters.
Here’s a good description: Balloon flowers
You say that you have them in a flower box: ideally, you would simply transplant them into the garden, so that they do survive the winter.
If you are growing them on a balcony, that’s another story. The issue with overwintering containers, is, as you suggest, a matter of how much soil there is, i.e. the size of the container. The bigger, the better.
Here’s our guide to Container Gardening.
Included in the helpful advice in the guide is a section on overwintering plants in containers, describing the freeze/thaw cycle that endangers them.
And congratulations on growing the balloon flowers from seed!