Here is how to fix it:
on cmd - create the presentation html by:
ipython nbconvert --to slides yournotebook.ipynb
that will create an html file. open the html file and go to the reveal.initilize() section.
In it, enter the desired width of the presentation such that it will fit the width of the dataframe, e.g.:
Reveal.initialize({
...
// The "normal" size of the presentation, aspect ratio will be preserved
// when the presentation is scaled to fit different resolutions. Can be
// specified using percentage units.
width: 960,
height: 700,
});