In Finder, directory size is not shown
In Finder press Cmd-J to access display options:



The option is set individually per folder. If you want it applied by default, click the button at the bottom. Calculation folder sizes make take a long time though for big folders.

Alternatively you can open Terminal.app, type du -sm (followed by a Space), drag the folder you want to know to size of into the Terminal window and press Enter. Takes some time as well and gives you the size in Megabytes.