The default width of the task pane in Office add-ins is quite small, making styling difficult when you want to display more content in it. At a 1366x768 resolution, for Excel desktop the width of the...
There's even a manifest entry all ready and waiting to be used: https://learn.microsoft.com/en-us/javascript/api/manifest/requestedwidth?view=excel-js-preview It "just" needs to be supported for task panes.