I’d say that it is definitely not just you that used the dimensions filter but, when it was there it did make choosing the correct size slightly more confusing for people who don’t care about pixel dimensions. It’s just another distraction for a regular user and makes the side bar filters appear less accessible “i don’t know what size I need so i’m not going to use these”. The small/medium/large thing is much simpler when it is on it’s own. Though it is going to make searching for mobile UI screenshots that little bit harder.
Ooh, also, here’s a handy hint that i’ve just found:
At the bottom of the filters on the left hand side there is a “show sizes” button. If you check this and then include the size of the image you want in the search term like this: ‘cars 1024x768’ most (if not all) of the returned images are of the correct size…might be handy