Resize the HTML createTemplateFromFile

Question

I have a modal displaying a HTML form. Modal is set to standard width. but i need to resize the modal to 900x900. After 2 days of study... not been able to do it.. please give me e solution (I'm a beginner)

//OPEN THE FORM retete IN MODAL DIALOG
function reteteModalDialog() {
var commesseAttive   = selArrashowFormInModalDialogyCommesse()
var elencoClienti    = selArrayClienti()
var elencoTipologie  = selArrayTipologie()
var elencoDimensioni = selArrayDimensioni()
var elencoVarianti   = selArrayVarianti()

  var form = HtmlService.createTemplateFromFile("selCommessa");
    form.arrayOfNames     = commesseAttive; 
    form.arrayOfClienti   = elencoClienti;
    form.arrayOfTipologie = elencoTipologie;

  SpreadsheetApp.getUi().showModalDialog(form.evaluate(), "Retete");

}

just change the size of the html Modal