I have tested further. And you can change the product card size also in Odoo V19 online. You can upload "static" modules, a zip file. This ZIP is composed with the following: the Manifest file is
{

    "name": "Custom POS Styling",

    "version": "1.0",

    "category": "Point of Sale",

    "summary": "Voegt aangepaste CSS toe voor grotere productkaarten in POS",

    "depends": ["point_of_sale"],

    "assets": {

        "point_of_sale.assets_prod": [

            "custom_pos_style/static/src/css/pos_custom.css"

        ]

    },

    "installable": True,

    "auto_install": False,

    "application": False

}
The assets view XML is

<odoo>
<template id="assets" inherit_id="point_of_sale._assets_pos" name="Custom POS CSS">
<xpath expr="." position="inside"><link rel="stylesheet" type="text/css" href="/custom_pos_style/static/src/css/pos_custom.css"/></xpath></template>
</odoo>

The CSS is

.pos .product-list .product {

    min-width: 170px !important;    /* breedte van kaart */

    min-height: 180px !important;   /* hoogte van kaart */

    padding: 10px !important;

    box-sizing: border-box !important;

}

/* grotere productnaam en beter leesbare prijs */

.pos .product-list .product .product-name {

    font-size: 14px !important;

    line-height: 1.2 !important;

}

/* Maak minder kolommen (= visueel grotere cards) — pas '3' naar gewenste aantal per rij */

.pos .product-list {

    display: grid !important;

    grid-template-columns: repeat(6, 1fr) !important;  /* 3 kaarten per rij */

    gap: 10px !important;

}

/* Zorg dat de knoparea / naam netjes centreert */

.pos .product-list .product .product-meta {

    text-align: center !important;

}

/* Optioneel: vergroot knopgrootte bij selectie */

.pos .product-list .product .button-qty {

    font-size: 16px !important;

    padding: 6px 10px !important;

}