Currently, there doesn't seem to be a quick way of knowing how many items are there in a collection. Of course, you can select all in the central pane and see the results in the right pane but that requires a couple of extra steps.

However, my suggestion is that the name of the collection in the left pane, should show the number of items in parenthesis, next to it. This will give a quick idea about the size of the collections in the left pane itself.

Thanks for providing such a useful software as free and open source. It is undoubtedly the best tool in the category and beats the hell out of other applications, even the commercial ones.