R markdown output size

I achieved it by adding my own css file. You can include it like this:

---
title: "Spielercluster"
output:
  html_document:
    css: style.css
runtime: shiny
---

Then you have to adjust the sizes of the class shiny-frame

.shiny-frame{
  width: 1400px;
  height: 1200px;
}