In FreeBSD, dvipdfmx and xdvi use libpaper which is not support JIS B series paper size, which is one of the standard paper size series. JIS B is like a ISO B series, but not the same.
The attached patch may fix the problem. If it not allow to add the patch in print/libpaper, I hope making japanese/libpaper to use the patch.
Created attachment 197046 [details]
a patch for print/libpaper to enable JIS B series paper size