When I use

@page {
size: 7.375in 9.125in;
-prince-trim: 0.9375in 0.5625in 0.9375in 0.5625in;
}

the resulting PDF shows a paper size of 8.66667 x 11.1667 in Okular. I don't understand why it isn't 8.5 x 11:

7.375 + 2 * 0.5625 = 8.5
9.125 + 2 * 0.9375 = 11

Where does that extra 1/6 of an inch come from?

And the answer is: It's the crop marks. When I remove

marks: crop;

I get 8.5 x 11.

Alternatively, removing 6pt each from the -prince-trim values does the trick.

I now see that https://www.princexml.com/doc/css-props/#prop-marks says: "Specifying crop and/or cross for the marks property will result in 6pt of bleed area being added to all four sides of the page and additional trim area to contain the crop marks."