Maybe you can use
a4paper to specify the paper size, and manually play with margins? For example, if you are using
B5, then the page width is 176mm. Assume you want 25.4mm margins on the page, this leaves 176 - (2 * 25.4) = 125.2mm width for text. The pagewidth is 250mm, so the total text height should be 250 - (2 * 25.4) = 199.2mm So you can set
in the preamble. Now, A4 paper has width 210mm, so the difference is 210 - 176 = 34mm. You want the odd pages on top left, and even pages on the top right, so you set
(since 25.4mm is added to the values of
\topmargin automatically). This should get you what you want.