Hi, 

To set the scrollbar for an one2Many field you just have to add width for the field, this can be achieved by adding a class on the field and styling the class in a CSS file, to attach a CSS file in odoo 14 the file should be mentioned in the assets XML file. The format for it is :

<?xml version="1.0" encoding="utf-8" ?>

<odoo>

<template id="assets_backend" inherit_id="web.assets_backend">

                         <xpath expr="." position="inside">

<link rel="stylesheet" href="/your_modulename/static/src/css/style.css"/>

                         </xpath>

                  </template>

             </odoo>

    in the CSS file:                 

​.your_class{ width: your_value px; }

    to set the scrollbar on height, set these attributes in CSS:                

​ .your_class{ height: your_value px;

​ ​ overflow: auto; }

Hope it helps