Text size makes all of ATOM bigger: text, buttons, everything. Settings > Look and feel > Text size.
ON THE ATOM PHONE APP: pick Match my iPhone, Standard, Large, Larger or Largest. Every screen gets bigger and still fits the phone, nothing runs off the side.
MATCH MY IPHONE (the default): ATOM follows your iPhone's own setting, Settings > Display & Text Size > Larger Text. Change it there and ATOM matches the next time you open it. This needs the newest ATOM app from the App Store; until you have it, Match my iPhone means Standard, so pick a size instead.
ONLY THIS PHONE: the size you pick stays on this phone. Your laptop and other devices keep their own.
ON A COMPUTER: use your browser's zoom, Cmd + on a Mac or Ctrl + on Windows (Cmd 0 or Ctrl 0 goes back).
IN THE ATOM MAC APP: Settings > Look and feel > Text size has Small, Standard, Large, Larger and Largest, and the keyboard works too: Cmd + makes everything bigger, Cmd - smaller and Cmd 0 goes back to Standard. The size stays on that Mac.
PRINTED DAY SHEETS AND PDFS are not changed by this setting.