I also wanted to request the option to select the assistant width.
As for now I use uBlock or Tampermonkey. As a reference for anyone interested here are the filters/scripts I have used:
uBlock
Spares 169px from the right to not clip under the Kagi logo in the top right corner:
kagi.com##:matches-path(/assistant).main-center-box > :first-child:style(width: calc(100% - 169px) !important)
Doesn't care about the logo (default behaviour for small windows):
kagi.com##:matches-path(/assistant).main-center-box > :first-child:style(width: 100% !important)
Tampermonkey
// ==UserScript==
// @name Resize Kagi Assistant to use full width
// @namespace http://tampermonkey.net/
// @version 2024-12-04
// @description Resize Kagi Assistant to use full width
// @author freezingDaniel
// @match https://kagi.com/assistant*
// @icon https://kagi.com/favicon-assistant-32x32.png
// @grant GM_addStyle
// ==/UserScript==
GM_addStyle('.main-center-box > :first-child { width: calc(100% - 169px) !important }');