The default canvas size of the desktop Classic form for Nintex Forms for O365 is height 600 pixels and width 700 pixels but this can be changed.
 

To change the size of the canvas for a Classic Nintex Form click the Properties option from the ribbon in the Nintex Forms designer.
This reveals the properties pane on the right hand side of the screen.
In the Properties Pane select the layout you would like to change the size of and select Layout Settings to open the Layout Settings dialog where you can adjust the height and width of the form.