看下面这张图片便一目了然！！！
借助 geometry 包，可以很方便地调整页面大小，常用的参数如图所示，这些参数都可以通过LateX支持的单位(mm, cm, pt, in)去重新设置。具体有以下参数：
- textwidth Corresponds to element 8 in the figure.
- textheight Element 7 in the figure.
- total Depends on other parameters, by default defines the dimensions of the Body, but can be combined with the includehead, includefoot, includeheadfoot and includemp commands to change the dimensions of Header, the Body, the Footer and the Margin Notes altogether.
- left, lmargin, inner These three parameters change the length of the left margin. Elements 1 and 3 in the figure, combined.
- right, rmargin, outer These three parameters change the length of the right margin. Elements 9 and 10 in the figure, combined.
- top, tmargin These two parameters represent elements 2 and 6 in the figure, combined.
- bottom, bmargin These two parameters set the distance from the bottom edge of the document to its baseline.
- headheight Height of the header
- footsep Separation between the bottom of the text (baseline) and the top of the footnote. Element 11 in the figure.
- footskip Distance between the baseline of the text and the baseline of the footnote.
- marginparwidth, marginpar Width of the margin notes. Element 10 in the figure.
The paper size can be set to any size you need by means of the command papersize={⟨width⟩,⟨height⟩}.
Let's see an example with some of the aforementioned options:
\usepackage{geometry}
\geometry{
a4paper,
total={170mm,257mm},
left=20mm,
top=20mm,
}
详细参考：ShareLaTeX。