Sort by file size is fine but sorting by image size would be a better addition. I envision something similar to Google images small, medium and large settings for different resolutions. This option could easily be used to determine photos that are suitable for a wallpaper among other things.

P.S Filtering by specific size (W x H ) would also be useful.

I agree, both for sorting by dimensions and filtering by dimension. I submitted wishlist items about both to bugs.kde.org already:
Add filters for image dimensions
Sort by image dimensions