If you add a full width header to a Divi Theme page, you may find that it looks right on desktops, but that the font sizes are too big on mobiles. This can be fixed by using one of Divi's more hidden features – different font sizes for mobiles and tablets.
If you go into the full width header's settings, you should find a setting link for changing the title font size:
If you change the value from the default, a little mobile icon will appear after it:
Now click on that mobile icon. It will bring up a modified version of the font size selector, which lets you set different values for mobiles and tablets:
Using this, you should be able to reduce the title font size so that it looks right on mobiles / tablets.
There are similar settings for the subheading and content text font sizes.