\version "2.11.61"

#(set-default-paper-size "a4")

#(define-public (printpapersize)
  (let* ((inside-paper? (module-defined? (current-module) 'is-paper))
         (default-paper (ly:output-def-scope (eval '$defaultpaper (current-module))))         
         (paper (if inside-paper? (current-module) default-paper))
         (size (eval 'papersizename paper)))
  (ly:message "The current paper size is: \"~a\" (inside paper ~a)" size inside-paper?)))

#(set-default-paper-size "letter")
#(printpapersize)
  
#(set-default-paper-size "a4")
#(printpapersize)

#(set-default-paper-size "a3")
#(printpapersize)

\book{
  \relative c' {c1} 
  \paper {
    #(set-paper-size "a2")
    #(printpapersize)
  }
}

% The papersize is reset to a3!
#(printpapersize)