There are two questions here:
1) How to specify the page size. This is best done with the geometry package, which has good documentation. The key idea is that you only specify as many parameters as necessary, and geometry fills in the rest. For example, you could say
\usepackage[twoside,papersize={7in,10in},margin=1in]{geometry}
and have the text width and height implicitly defined.
2) How to place crop marks that delineate your user-defined page on the A4 paper. For this, you can use the crop package, for example
\usepackage[cam,a4,center]{crop}
This package also has good documentation. Both packages are part of TeX Live so most likely you already have them installed.
I'm quoting this answer from stackoverflow:
For debugging page layout, Peter Wilson's layouts package is the way to go. Here's an example for visualising the page design and seeing the dimensions used for it
\documentclass[a4paper,12pt]{report}% I didn't add the twoside option in order to only display the right page layouts
\usepackage{layouts}
\usepackage{geometry}
\begin{document}
First you see the default page layout:
\currentpage
\pagedesign
\clearpage
\newgeometry{text={155mm, 24cm}, top=25mm}
Now you can check the page layout you passed to geometry:
\currentpage
\pagedesign
\clearpage
\newgeometry{inner=16.5mm,outer=11mm,top=25mm}
and then check the possible layout you expected (I'm assuming you're using A4 paper)
\currentpage
\pagedesign
\clearpage
\end{document}
As you can see (from the first layout displayed), the default page layout for report (and the other default classes), as @egreg pointed out, does define an outer margin larger than the ineer one.
Since you jest passed to geometry the format for the body (with the text={<textwidth, textheight>} option), it doesn't change the page's default, just adjusting itself to it. In order to achieve what you expect, you must tell geometry what is the inner and outer margins lengths you expect. You can do that both with inner and outer options or with left and right (or lmargin, rmargim, check geometry doc, p. 9)
EDIT: Just updating, following @daleif's comment:
It's considered good typographical practice to use larger outer margins and smaller spines — for the sake of tradition or just to provide the reader a space for personal notes. There's a a wonderfully complete answer explaining that, by Yannis Lazarides (Actually, that question is closely related to yours).
Best Answer
Maybe you can use
a4paperto specify the paper size, and manually play with margins? For example, if you are usingB5, 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 setin 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
\oddsidemargin,\evensidemargin, and\topmarginautomatically). This should get you what you want.