@deborahcreekmore there is no hard rule here but the area seems to be about 540px wide, you can make it as high as you want the text will just go down… 

The screen-grab I am sharing here is for the 404 instead of 403 but the pages are roughly identical. And as I’m sure you’ve seen the phrases support HTML so you can resize a large image as needed.

For the 403 the code we used is:

<p><img src="INSERT IMAGE URL HERE" alt="" style="display: block;  margin: 0px auto 20px auto;" height="400" /></p>

<p style="text-align: center; font-size:16px;"><strong>Sorry, the page you are looking for might have been removed, is temporarily unavailable, or the name has been changed.</strong><br></p>

Hope this helps!

Thank you, @SmartlyGreg This is very helpful!!