Page settings allows you to adjust the overall dimensions of your score such as page size, page margins, and scaling. It is one of the main layout tools in MuseScore—along with the options available from Style→General... Access from the menu, Layout→Page Settings...
Here you can select the paper format, either by standard name (e.g., Letter or A4), or by specifying the height and width in either mm or inches (use the radio buttons to choose which unit of measurement to use). The initial default page size depends on your localization—in the United States, Letter size paper is standard.
You can also choose to format your music in Landscape or Portrait orientation using the radio buttons. Prior to version 2.1 unchecking Landscape enabled Portrait format. You can optionally use Two sided layout (i.e., book format, with mirror left and right margins for even and odd pages—see below).
Odd/Even Page Margins
The Even Page Margins and Odd Page Margins settings allow you to define the printable area of your pages. Aside from changing the margins around the music on the page, other settings, such as the positions of headers and footers, are calculated relative to the margins defined here.
If the "Two sided" checkbox under "Page Size" is selected, you can set margins differently for mirroring odd and even pages. Otherwise, only one set of margins can be modified, but will apply to all pages.
To display page margins in your score on screen (though not in print), go to View→Show Page Margins.
The Scaling property allows you to increase or decrease the size of your score.
In MuseScore, the sizes of score elements, such as note heads, note stems, accidentals, clefs etc., are defined in terms of a unit of measurement called a staff space (abbreviated to "sp"). One staff space is equal to the space between two lines of a music staff (or one-quarter the size of the full five-line staff).
As you change the "Staff space" setting (under Scaling), all score elements follow suit and thus correct proportions are maintained. The exception is Text in which you can set an absolute value, independent of "Scaling."
Note: Changing the "Scaling" does not always change the number of systems per page, because system distance can vary between limits set under "Min system distance" and "Max system distance" (see Style→General...→Page).
First page number
Sets the number of the first page of the particular score. Page numbers below 1 won't get printed—e.g., setting the first page number to -1 would result in the first and second page showing no page number, and page number 1 appearing on the third page.
Apply to all Parts
The Apply to all Parts button is available when modifying a part, rather than the main score (see Part extraction). If you change the page settings of one part and want the rest of the parts to have the same settings, this button will apply the change to all parts in one go.