Two documents have the same page format if width and height of all pages have the same values. The tolerance defined by DIN 476 is taken into account:

@Test
public void haveSameFormat() throws Exception {
  String filenameTest = "documentUnderTest.pdf";
  String filenameReference = "reference.pdf";
  
  AssertThat.document(filenameTest)
            .and(filenameReference) 
            .haveSameFormat()
  ;
}

The comparison can be restricted to selected pages:

@Test
public void haveSameFormat_OnPage2() throws Exception {
  String filenameTest = "documentUnderTest.pdf";
  String filenameReference = "reference.pdf";
  PagesToUse page2 = PagesToUse.getPage(2);
  
  AssertThat.document(filenameTest)
            .and(filenameReference)
            .restrictedTo(page2) 
            .haveSameFormat()
  ;
}

@Test
public void haveSameFormat_OnEveryPageAfter() throws Exception {
  String filenameTest = "documentUnderTest.pdf";
  String filenameReference = "reference.pdf";
  PagesToUse pagesBefore2 = ON_EVERY_PAGE.before(2);
  
  AssertThat.document(filename)
            .and(filenameReference)
            .haveSameFormat(pagesBefore2)
  ;
}

All possibilities to select pages are explained in chapter 13.2: “Page Selection”.