How to get full size exports in Photos?

Léonie, here I am again, an illustration of the issue.

I have an album and I'm displaying thumbnails. The first picture is at top left, and it has these characteristics in Photos:

I then export it:

and it ends up with the same pixel size as expected:

I then pick the next picture in the album (top row, second from left), which has these characteristics in Photos:

I then export it, exactly as before:

And I get this:

As you see, the pixel size is reduced (just a bit, in this case)!

What can be the reason for this? Should I call Apple?

Good Lord - it was as simple as that! I was not expecting the disclosure triangle to do anything related to image size. It is very strange that simply by clicking on the triangle, the default option becomes full size!

Anyway, solved, and thanks very much!

Tragically, this doesn't always work. For example, I have an album in which the first photo in the row is 2769 x 1536 pixels. If I export this to a folder on my iMac, using the Export dialog as described earlier in this thread, I get the same size - fine. The next picture however, is 1900 x 1098 pixels. Exporting it using exactly the same technique yields a picture sized 1166 x 674. Subsequent photos also suffer in the same way, This happens when I try to export individually or when I try to export a group of pictures for the same album. It doesn't seem to make any sense.

I thought I knew one workaround, which was to edit the picture in an external editor. However, when I try this, using even the rock-solid Preview, I get an error:

"The operation couldn’t be completed. (PAImagingErrorDomain error 1019.)"

I really have no idea what's going on, but my pictures, which I really need in the highest resolution possible, appear to be trapped inside Photos!

Can anyone help?

Thanks Léonie. I thought I had already answered this but I can't see my answer, so here is a modified version again. I am using JPG throughout.

I can export the unmodified original, but really that's a waste of all the editing I've done in Photos. I have now found that exporting an edited photo as PNG seems to work (pixel size is preserved), but of course the PNGs are much bigger than JPGs, so I have to open them in an editor and re-save them as JPGs (Preview will do it). I have no idea if I am introducing more loss by doing this, but it will work for the project I'm doing now (photos to show a property for sale). However I don't see it as a permanent solution, and I still don't know why the original issue arose. Should I contact Apple Support do you think?

Well, I have rebuilt the library and restarted a good few times. I will run a few more experiments to make sure it isn't an intermittent problem. Back soon.