header image for Osiris theme

try rather this:

background-size: 100%;

i new with wordpress edit theme , i hope explain to me in details how i can do that ?

add this to Custom CSS,

#branding {
background-size: 100%;
}

Here is an alternate solution to beautify your page:
https://postimg.org/image/ai5qo0mab/
Add this to Custom CSS

#content .top-content {
float: none !important;
margin: 0 auto!important;
}

Have you tried this for 100% width

Dashboard> Appearance >osiris Theme Options

Go to Miscelleneous and thereafter tick this checkbox:
Right Sidebar
Remove Right sidebar from all the pages and make the site full width