In the css of your modal popup, set the width of the modalpop up, then set the overflow:auto. That will give you horizontal scrollbar. Example:
So,when the content width exceed the 900px, the horizontal scrollbar will show up.
The same is true for the vertical scrollbar where you need to set the height of the modalpop.
That solve the issue if the browser screen is at least 900px. For a solution with a popup that could span to fill that page, see my Answer.