To view the folder size, go to the main menu (File –> Folder size) or press the space bar.
You can view the folder size even inside archives, DropBox or on an FTP server!
The option works only for folders, not for bundles. The size of bundles is counted automatically in the background. For example, if you go to the Applications folder, it will take some time to count and display the size of each bundle. If you leave the folder, the counting of the size will stop, and this value will be saved until you restart the program.
Showing folder size works in Full View mode only:
To view the size of the folder and of all its contents, you can also click the Get info toolbar button (or the Command+i hotkeys).