fit the widht

Hi
sorry but I don't really understand what you want to do here ...

he fullwidth menu seens to shrink to the size defined by the menu items.

the fullwidth will take the whole with of the menu bar, you can make it fullwidth of the page with some css adjustment (depending on your template also)
CEd

In the attached image you can see that the fullwidth menu is not actually full width. Maybe I have the settings wrong somewhere?

as said

the fullwidth will take the whole with of the menu bar,

and

you can make it fullwidth of the page with some css adjustment

give me your url to check

add this in a custom css
it will take the whole width of the container #top in the template
as this #top is positioned in absolute, it is used as reference for the submenu width.
unless you make some modifications in your template to give a natural positioning to #top, you will not be able to set the submenu fullwidth in a natural way

That worked great for the submenus, but it did not cover the top level.
I also tried changing:
/* container style */
div#<?php echo $id; ?> ul.maximenuck {
clear:both;
position: relative;

to:
/* container style */
div#<?php echo $id; ?> ul.maximenuck {
clear:both;
position: static !important;

This had the same affect as adding:
div#maximenuck343 ul.maximenuck {
position: static !important;
}

But neither altered the first level.

all this has to be managed in your template and is not related to the maximenu module
it float right because of in
templates/mak/css/template.css
line 792