Hi,​

I think there is no such feature to add window size from python code

you can change window size by extending Dialog widget:
​

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <template id="assets" inherit_id="web.assets_common">
        <xpath expr="." position="inside">
            <script type="text/javascript" src="/custom_module/static/src/js/dialog.js"/>
        </xpath>
    </template>
</odoo>

dialog.js :-

odoo.define('custom_module.small_dialog', function (require) {
"use strict";
var Dialog = require('web.Dialog');

Dialog.include({
    willStart: function () {
        var self = this;
        if(this.title === 'Create new interest note'){
            this.size = 'small';
        }
        return this._super.apply(this, arguments);
    }
});

});

You can also use 

this.size = 'medium'; 

by adding necessary changes in the code.

Thanks