Start by measuring your available space—aim for a desk 100 to 120 centimetres...

https://connersinterestingblog.theglensecret.com/when-does-a-second-monitor-actually-help

Start by measuring your available space—aim for a desk 100 to 120 centimetres wide to fit your gear comfortably. Use a monitor arm to free up desk surface and create more room for your keyboard and mouse