Google Drive lacks a very basic feature: calculating folder size. There is no solution in the web interface to view the total size of a given directory. There are a couple of dubious-looking online “folder size analyzers” that request access permissions to your Google Drive and offer you the basic functionality of calculating folder size. While those apps have a legitimate need for access permissions to your account, you may consider giving those permissions to random apps a questionable decision.
The (very) useful tool rclone, which provides an rsync-like interface to many cloud storage providers, implements this functionality. After configuring rclone to work with your Google Drive, use the size command to determine the size of a given folder:
$ rclone size "gdrive:Pictures/" Total objects: 1421 Total size: 9.780 GBytes (10501374440 Bytes)
The size is calculated recursively. However, there is no simple way to display the size of each subdirectory recursively.