Hi there,

Actually the size of the cover photo is depend on your content's width which is varies in each template you've using for an example(on my site using default Joomla template) http://take.ms/HNIXZ

//change those width accordingly.

#es .es-profile .es-profile-header__hd.with-cover{

width:830px !important;

}

Please give it a try and see how it goes