If you were using Windows XP, you could use the Folder Size shell-extension which gives you a few columns that you can add to Explorer to show the size of a folder, as well as the number files/folders/both contained therein, thus allowing you to view and sort them right inside Explorer.
Unfortunately, Vista and higher have dropped the API functionality that Folder Size and similar programs use, so they no longer work.
Instead, you’ll have to use a non-shell-extension. Fortunately there are several programs that can do a pretty good job of visualizing space usage on a file-system.
Most of them have a few different modes including text-list/tree (like in Explorer), pie-graph, and treemap, in which the files and sub-directories are represented by squares or rectangles. There should be an option to display the number of files/folders in addition to the size, usually in text-list/tree mode. Sometimes, to get the number of files/folders, you need to select the folder and view its properties (which is less convenient, but still easier than manually in Explorer).
Here are some of the most popular ones (some freeware, others shareware):