I would like to make my featured images display in larger size.

Logically I'd expect 'ATO > post thumbnails' to let me adjust the height and width sizes. But changing the size isn't working.

Should I be adjusting something elsewhere?

I believe there is an interaction between the Atahualpa thumbnail settings and the WordPress thumbnail settings in Settings>Media. Be aware that if you make changes in Settings media this will only affect images uploaded after the change. In order to affect images already uploaded you will need to use a thumbnail regeneration plugin or delete and reupload those images.

You can use the atahualpa settings to reduce the thumbnail size specified in the dashboard->settings->media but you cannot increase the size of the thumbnail via atahualpa.

So you can make them smaller with the atahualpa setting, no bigger.

Thank you - even if I don't manage to make my featured images bigger, I just learned how to control the max size of images imported to media library which is just as useful.