Dear Philipp,
Yes, in the fourth parameter: Dialog.open(url, args, callback, options)
A code will be like this:
Dialog.open("https://domain.sharepoint.com/sites/...",{}, function(){}, { width: 500, height: 500 });
The description will be in the docs soon: https://plumsail.com/docs/forms-sp/javascript/dialog.html#intro