Getting it onto one page

Four settings change how much fits, and one of them is not a setting at all. The numbers below are the limits the typesetter enforces, not rules of thumb and pushing past them pulls the value quietly back.

The three you can drag

In the editor these are sliders, and the page redraws under them. The count at the top of the document is measured with the same model that typesets the PDF, so “fits one page” is a measurement, not a guess.

Top and bottom margin0.30in – 1.20in: The biggest single lever, and the first one to reach for. Below about 0.4in a page starts to look crowded rather than full.
Left and right margin0.30in – 1.20in: Buys width for every bullet at once, so it removes wrapped lines rather than shrinking them.
Text size8.0pt – 14.0pt: Moves in quarter points, because that is the grid the fitter picks on. 10pt is the default; below 9pt is legible on screen and tiring on paper.
Line spacing0.80× – 1.50×: The subtlest of the four. Section headings do not follow it (they are fixed by the document class), so tightening this compresses body text only.

The fourth one: cut something

There is a point where the sliders stop helping and start making a cramped page rather than a shorter one. Past it the only real move is removing content: the oldest role, the bullets that describe duties rather than outcomes, the skills nobody asked for. In the editor a section can be dragged out of the way or hidden without being deleted, so this is reversible.

And one page is not automatically right. Two is normal past about a decade of work, and a resume squeezed to one page by shrinking everything reads worse than a comfortable two.