The Windows Documentation has some confusion around the two different JIS and ISO B paper types.
DMPAPER_B4 B4 (JIS) 250 x 354 mm DMPAPER_B5 B5 (JIS) 182 x 257 mm DMPAPER_ISO_B4 B4 (ISO) 250 x 353 mm DMPAPER_B6_JIS B6 (JIS) 128 x 182 mm
i.e. DMPAPER_B5 is documented as JIS and the documented dimensions match that format. While DMPAPER_B4 is then documented as JIS but the documented dimensions match the ISO format B5 of 250 × 353 mm. Given the presence of DMPAPER_ISO_B4 and the absence of DMPAPER_B4_JIS I’ll bet that it is really the JIS size of 257 x 364, which is backed up by the PPD Specification stating that the Microsoft DMPAPER_B4 size is 257×364 and by the Microsoft Word UI which lists B4 (JIS) with (correct) dimensions of 257×364.
Turning our eyes to the Excel Documentation we have more confusion with the XlPaperSize enumeration. Here we have…
xlPaperA5 A5 (148 mm x 210 mm) xlPaperB4 B4 (250 mm x 354 mm) xlPaperB5 A5 (148 mm x 210 mm)
Well what size is xlPaperB5 ? Is it truly ISO A5 (148 mm x 210 mm) like the comment says. Or might it be ISO B5, i.e. 176 × 250 mm. Or, seeing as the XlPaperSize enumeration has the same values as the DMPAPER values for all other values, are both xlPaperB4 and xlPaperB5 actually the same values as DMPAPER_B4 and DMPAPER_B5, which we’ve earlier determined are JIS B4 and B5 sizes, and not ISO sizes at all. That’s the most likely answer, i.e. the text is completely and totally wrong for xlPaperB4/xlPaperB5