I assume that when you say that Xcode doesn't allow to change the size you mean the interface builder.
If my assumption is right you should change the "Simulated User Interface" of your Viewcontroller in interface builder. Select the root view and set Bottom Bar to Tab Bar in the attributes inspector. This changes the size of the view to 320x411 without touching the raw size attributes.

EDIT: I wouldn't recomment it, but if you really want to change the size of the view in interface builder, you have to disable all elements from the simulated user interface.
I guess it's the simulated status bar that prevents you from resizing the view.