Within the Berlin category on Wikimedia Commons, I want to see all images with width > 2100 pixels.
Is there a way to get this?
Bonus if thumbnails are shown, so that I can efficiently choose a picture I like.
You can sort by size in catscan (by Magnus Manske, the creator of MediaWiki): http://tools.wmflabs.org/catscan2/catscan2.php
Berlin images sorted by size; or, more useful category, Quality images of Berlin sorted by size. You can't filter by width, but file size is a good approximation, there's a width column and you can pick the appropriate category. Walking subcategories is unreliable due to how MediaWiki categories are made but you can try (look at the bottom after all the errors). You can also produce a gallery which you'd then need to preview on a Commons page; and there are other tools/versions which produce thumbs but have less filtering/sorting options.