You are right: One can only see the size of a single file (via properties).
Result
There is no way to see the size of a folder or multiple files.
(Workaround: Download your CryptDrive to use your local OS size tools.)
Estimation
IMHO it's self explaining, that this feature is necessary some time in the future, especially if users pay for storage. My guess is, that cryptpad.fr grants so much storage that their users don't run out of space yet. Further I think they will implement this feature as soon as some time in the future more and more of their users ran out of space and ask how to solve this. Personally I don't expect this feature earlier.
Future
When this feature gets implemented I think the easiest (aka cheapest) way would be implementing a properties dialouge for folders. I guess that this would be enough to analyze storage usage and that the alternative - showing the added sum of multiple marked files - would be technically more complex (aka more expensive).
Folder Properties
A folder properties dialouge is sustainable for the future development of CryptPad. The first logical evolution would be to later remvoe the folder color dialogue and instead implement it as part of the folder propertis dialogue. Further logical trends would be showing creationand last access time and later expanding the tagging system of files to folders.