No. Because of CSS Isolation, you can't generally monkey with the internals of a component, especially if it's in another namespace. The documentation says:
CSS styles defined in a parent component don’t leak into a child. In our example, a p style defined in the todoApp.css style sheet doesn’t style the p element in the c-todo-item component, because the styles don’t reach into the shadow tree. See CSS.
There is currently a loadStyle hack that works around this, but I expect it will eventually be fixed.
You should be able to specify a height for the entire table, which should auto-size the inner table accordingly:
This is because the Shadow DOM allows us to modify the "outside" of the component, such as its size, a border, etc, while not allowing us to modify the internal components.