Open a new Terminal Window and type the following command:
du -shc folderPath
Explanation of the du command:
du - estimate file space usage -s, --summarize, display only a total for each argument -h, --human-readable, print sizes in human readable format (e.g., 1K 234M 2G) -c, --total, produce a grand total
Replace folderPath with the folder to check the size on.