How can you upload a logo which has the whole width of the site?

Hello Julia !
To increase size of logo in header (E.g: 450x70px), you can do as the following:
Step1: Log in to Dashboard > Appearance > Widgets > Header, delete all the Text widgets in the Header section. 
Step 2: Go to Dashboard  > Appearance >  Customize > General Settings, add the following code to the Header Code:

   <style>
 .site-header #header {
    background: none;
}

#branding {
    width: 450px !important;
}

.site-header #branding a {
    width: 450px;
    height: 70px;
}
</style> 

Notice: You can change the width & height in the code.
– To make logo bigger on the mobile/ ipad version please add following code to Header Code in Dashboard  > Appearance >  Customize > General Settings

   <style>
@media (max-width: 979px) {
 .wrap-navigation .small-logo {
 webkit-background-size: auto 100%;
 -moz-background-size: auto 100%;
 -o-background-size: auto 100%;
 background-size: auto 100%;
 }
}
</style>

Hope this helps !