This Requirement Is New
ESTA collected no image for most of its life.
For years the honest answer to "what size is an ESTA photo" was that there wasn't one, because the application did not collect an image at all. Plenty of guidance still says exactly that, and it is now out of date.
CBP's official ESTA site lists what you need to apply, and the list reads: a valid passport from a Visa Waiver Program country, a traveler's selfie photo, a valid email address, a home address and phone number, an emergency contact, and a payment method.
So a photo is part of it now. If you last applied a few years ago and remember it as a purely text form, that has changed.
