is there a way to set a custom zoom size in pages

While you cannot "set" a custom zoom size, you can produce a custom zoom scale by choosing "Fit width" from the pop-up menu in the Zoom button, then adjusting the width of the document vs the width of the window.

Drag the document's title bar to the left to place the left margin outside the left side of the screen, then (with both left and right sidebars closed) drag the right edge of the document margin right to increase the zoom, leaving about as much width of the right margin showing as you have hidden text off screen to the right. he drag the title bar right to centre the document text in the screen.

Full width on a doc with 1" margins on a Letter size page showed this zoom value (when dragged far enough right to show the zoom button):

"Fit…" values are not saveable (as settings) though, nor will the zoom setting be retained when the document is Saved and later opened.

Regards,

Barry

In Pages Preferences : General, there is a selector for Zoom that by default is set to 100%. This will apply to any document, and there is no per-document zoom setting.