Custom Max Width/Height for photo gallery?

Do you mean the standard “gallery block” or something else?
Essentially, the detection is done based on how/where you upload images. If the photo gallery you reference just uses the standard upload interface (in the post editor or otherwise), then there is no way to differentiate it from an image inserted individually into a post.

Thanks for your reply. I am using a gallery plugin, but it does appear to use the standard upload interface.

What I’m seeking is for the photos in the galleries to have a certain maximum height and width, but to have Featured Images for pages have a different maximum.

For now I’m planning to set Page/Post to the value I need for the photo galleries, and then set Media Library to the value I need for the Featured Images (which will be full width and should equal the largest images I’ll need anywhere.) Then after the Featured Image is uploaded to Media Library, then the user can go to the Page and insert the image. I think that will work.

Thanks for your continued support of this plugin. I’ve been using it for a number of years and I’m glad you took it over and kept it going.

That should indeed work! I double-checked, and there are 2 different methods used by the block-editor for uploading images (REST API & async-upload.php). The latter is used when adding featured images, but both methods are used for adding images to a gallery, making it impossible to tell the difference.
So your route is likely the best solution, hope that works out well for you!