Hey @mariusnicula!
First, you can always add your own css to the admin-side of WordPress. With that you can adjust the styles however you’d like. Just use the admin_enqueue_scripts filter.
Next, it’s intended for groups, but even outside of them you can use the 'columns' attribute in fields to fit your fields within a 12 unit grid. So 'columns' => 12 is full-width.
Finally, for total customization, check out field templates.
Hope this helps!