Pages image size?

You can scale an image to the 7.5 inch width, and then from the Format menu : Advanced : Define as Media Placeholder. Any future image that you drag and drop on that media placeholder will automatically resize to the dimensions of the placeholder. Before you make the media placeholder, ensure that you have disabled Text Wrap on the selected image.

Otherwise, there is no user configuration in any version of Pages that automatically resizes the dropped image to a specified size, other than the default page width.

Pages will fit images between margins, so what you have observed is only a fit, not a same size import. You need to do experiments. That is how we discover what is going on.

I have done this particular experiment multiple times, to every version Apple has delivered, in the hope that finally Apple will have used some rational method to how it imports images.

In Pages 6.2 (you haven't said what you are using) the images will import accurately if they are 72dpi bitmaps or vector pdfs, otherwise the results vary according to a rule I have not cracked but suspect it assumes multiples of 72dpi in resolution.

Peter

When I create an exact, 468.0 pixel wide, 300 dpi vector rectangle in Affinity Designer, and export selection as 100% quality jpg; when I drag/drop it onto Pages v6.2, it does not automatically align it on the 1", and 7.5" boundaries for a document with 1" L/R margins. Using U.S. Letter.

Click the original size, and then scale down from there with the Size controls, which do allow you to directly enter the value that you want, if the incremental adjustments are too aggressive.