It all depends on what effect you want. This is the width and height of your text-box. Above these inputs is the position of the text box on the screen and below is the position of the text inside the text box.
My advice would be to try playing around with them to see what they all do.
I’m assuming the text box should be the same size approximately the same size as inserted photos and videos. I was making the last number 720 but it would default to 1280.
An important factor is whether you have the box “Use Font Size” checked. If not, the text will change size as you change the size of the text box - that is essentially what the numbers on the left are doing, setting the size and position of the text box. If you check “Use Font Size,” the font will stay the same size, but will move around as you change the numbers. But if you uncheck this box, the font will be set to the largest value that lets the text just fit in the box.