kabalrs
If you like to make changes to core files, here are some settings that will give you a full horizontal view, if you do make changes, make sure you have a good backup and keep track, in case you need to go back to default settings

Agent side:
/* My added CSS settings for file: /scp/css/scp.css /
/ To change from width: 960px to 90% line: 127*/
#container {
width:90%;
margin:0 auto 20px auto;
}

/* added width: 100%; line:517 */
table.list {
width: 100%;
}

/* To change Overflow Menu line: 727 width: 960px to 90% */
.jb-overflowmenu {
width: 90%;
}

/* added width: 100% line:1128 */
table.list {
width:100%;
}

/* added width:100% Line:1287*/
.form_table {
width: 100%;
}

/* added width: 100% line:1474*/
.ticket_info {
width:100%;
}

User side:
/* My added CSS settings /assets/default/css/theme.css/
/ Line:89 added width: 100% */
table {
width: 100%;
}

/* Line: 292 Change width from 840px to 90% */

#container {
width: 90%;
}

/* Line: 795 added width 100% */
#ticketTable {
width: 100%;
}

Change width of input for Replies
the file is \js\redactor-osticket.js and you change line 378:
from 'maxWidth': el.hasClass('fullscreen') ? '950px' : false,
to 'maxWidth': el.hasClass('fullscreen') ? '100%' : false,