Image size - EPUB reflowable

Shoot, I was sure I posted a summary guide to this, but I can't find it.

You can use any size image in InDesign, as you will usually be exporting to EPUB with image scaling enabled. Everything there depends on the export resolution, usually 300ppi (which Amazon/KDP more or less requires, not for entirely justifiable reasons) or 150ppi for most EPUBs and Kindle if you choose to go rogue.

Since reflowable e-books are inherently flexible, there isn't always a fixed answer. You have to estimate the screen size of the image, usually using 144ppi as the display value (that is, an image two inches across would be about 288 display pixels... which are not actual device pixels). Then you work backwards through your export PPI setting to determine a minimum size for the image in ID.

If you want an image to be three inches across, or about 432 pixels, the image in ID has to be at least that many pixels across, and up to double is generally a good thing. You usually don't want to have images much larger than needed, as it just clogs up file size and slows export times. But with the export scaling applied, any larger image will work. (Smaller images may be upscaled, which will reduce quality a lot — avoid this.)

As for how the various export settings (global and individual) affect the export size, here's a chart:

Image is originally placed size, at 4.5 inches across.

A:Width=1 inch;

B:Default;

C:Default 2X [CSS multiplier used];

D:Height=3 inches;

E:Width=50%;

F:Text Size;
G:Text Flow;

H:None (cropped at dotted line).

Take it from there.