Telegram has a very stupid restricted layout. This userscript removes all width restrictions. Also works with rambox.

telegramlayoutunrestricter.user.js

// ==UserScript==
// @name          Telegram - Full Sized Windows
// @namespace     http://userstyles.org
// @description	  Telegram has a very stupid restricted layout. This removes all width restrictions
// @author        Roman Karwacik
// @run-at        document-idle
// @match         https://web.telegram.org/*
// @version       1
// ==/UserScript==
(function() {
    var css = [".im_send_form {max-width: 10000px;}",
    ".tg_head_split {max-width: 10000px;}",
    ".im_page_wrap {max-width: 10000px;}",
    ".im_message_wrap {max-width: 10000px;}",
    ".im_send_panel_wrap {max-width: 10000px;}",
    ".im_history_col_wrap {width: 75%;}",
    ".im_dialogs_col_wrap {width: 25%;}",
    ".composer_emoji_tooltip{margin-left: -245px;}"];
if (typeof GM_addStyle != "undefined") {
    for (var i = 0; i<css.length; i++) {
        GM_addStyle(css[i]);
    }
} else if (typeof PRO_addStyle != "undefined") {
    for (var i = 0; i<css.length; i++) {
        PRO_addStyle(css[i]);
    }
} else if (typeof addStyle != "undefined") {
    for (var i = 0; i<css.length; i++) {
        addStyle(css[i]);
    }
} else {
	var node = document.createElement("style");
	node.type = "text/css";
    for (var i = 0; i<css.length; i++) {
        node.appendChild(document.createTextNode(css[i]));
    }
	var heads = document.getElementsByTagName("head");
	if (heads.length > 0) {
		heads[0].appendChild(node);
	} else {
		// no head yet, stick it whereever
		document.documentElement.appendChild(node);
	}
}
})();