Your file size is displayed to you on the collection information page, in the files area and in more detail from the file information page.
From the collection information page, scroll down to where you can see the uploaded files. The compressed size is displayed next to each thumbnail.
Click on any of the thumbnail images and you will see the file information page. The area "Size Info", will display the file sizes, aspect ratio, and suitable print sizes that match the aspect ratio.
You may also search for images that are small files in a collection! From the collection information page, scroll to the collection toolbar. Hit the search button:
From the drop down lists select the size option, aspect ratio or any other filter you would like to apply:
Then click "Search My Images". The images will now be shown in the order requested in the filter.