Size of HTML "page" when `_repr_html_()` is used?

I want to implement my own way for a custom object to render itself as HTML in a notebook.
For this I implemented _repr_html_().

The HTML I return is made to adapt to the window size of wherever it gets displayed.
I could not find any documentation of what the size will be for getting rendered in a notebook. I assume the width is just the cell width, which in turn depends on the size of the browser window the notebook is shown in. That is fine.

But what is the height? How is the height of an output cell that contains HTML determined?