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

Instead of the new timer joining as a separate Zoom participant, it is available directly in your Zoom application and can be shared through your video display. There are a few display options (shown here); just make sure you select "share timer display."

If you like how Meeting Timer Classic takes up a whole box in Zoom, you can select "only timer." This will mean that you are no longer visible, and the timer is taking up your entire video screen.

If you want both your video and the timer to be visible, you can select split screen, one of the corner options, or you can join your meeting from a second device. For example, you can join your Zoom meeting on a computer, set the timer to Only Timer display, and also join your meeting from a mobile device. You would then show up as two separate participants in the meeting, one with timer displayed and one with your own video shown.

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