If you do more than 1x, you'll probably want to have two separate bowls: 1) A big mixing bowl, where all the custard is mixed together, and 2) a small wide soaking bowl, that only needs to hold enough custard to soak a couple pieces of bread at a time.
You'll probably also want to add the cinnamon in after the custard is in the soaking bowl, and periodically add more after the departing bread has pulled enough away.