Hey merlieon,
Sorry for my late reply. So, to my understanding, you are wanting the gallery with one image set to full screen width?
One problem I can see is the height setting. A percentage height will not work with the gallery. If you want a fixed height, you will need to specify a pixel value (e.g. 500px). If you do not specify a height, then the image will maintain its natural proportions.
The other consideration is, if you are enlarging an image, that image may look blurry. This cannot be helped, as the original image is lacking the information required to enlarge it. Reducing an image’s size is fine, but enlarging it can cause issues.
However, WordPress automatically creates smaller thumbnails of your images. You can specify which size of thumbnail to display by using the quality parameter (e.g. quality="medium", quality="large", or quality="full").
Finally, if you do specify a fixed height, you can use crop_images="true" to keep the image from appearing stretched. However, once again, a percentage height will cause errors.
I hope this has helped. If not, get back to me.
Closing due to inactivity