Full screen nav menu block with avada theme

Select the Full Width Mega Menu on/off selector in Avada settings.

Here’s the steps to follow covered on that url:

How To Set The Mega Menu To Full Width

Step 1 – Go to WP Dashboard > Appearance > Menus, locate the first
level menu item and click the Avada Options button.

Step 2 – Once the settings window has opened, locate and activate the
Fusion Mega Menu & Full Width Mega Menu settings as seen in the
example image for this section.

Step 3 – Click ‘Save’ to close the Avada Options window and scroll to
the bottom and click “Save Menu” to save your changes made to the
menu.

I looked at your site code and your menu has a width of 960px set; Setting it to Full Width should fix this.