I want the new Timer to take up an entire video box like Meeting Timer Classic.

The new Timer cannot join as a separate participant, but to mimic this, we recommend trying Immersive Display or My Video -> "Only Timer" display method.

In Display Method, choose Immersive, select top or bottom, and then click "Share Timer Display." This will show the Timer as a separate box either smaller up top or larger down below:

Alternatively, you can select My Video display method and choose "Only Timer." This will mean that you are no longer visible, and the timer is taking up your entire video screen:

More details on the differences between the two timers can be found here.