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.