Create new notebook at fixed size

If you don't want to modify MenuSetup.tr you can evaluate:

SetOptions[
 $FrontEnd,
 NotebookEventActions :> {
   {"MenuCommand", "New"} :> CreateNotebook["Default"]
   }
 ]

which will add this to user's FrontEnd/init.m

"MenuCommand" example

If one feels comfortable modifying MenuSetup.tr, replacing its seventh line with

            MenuItem["&Notebook (.nb)", KernelExecute[CreateNotebook["Default"]], MenuEvaluator->"System", Method -> "Queued", MenuKey["n", Modifiers->{"Control"}]],

will result in Ctrl+n creating a new notebook with a window size of CurrentValue["DefaultWindowSize"].