iCloud storage is full, how to find each folder size?

Well, you could go to System Preferences > iCloud > Manage, and see which files take the most space.

You can see the file sizes in Finder! If you select iCloud Drive in the sidebar and get info on an item (select item and press cmd+i), it will show how much space it takes up in iCloud and how much space it takes up on your disk.

To make it easier to find the largest folders, make it show in List view, press cmd+J to open the View Options panel and enable "calculate all sizes". This makes it list sizes for folders as well as files, so you can see at a glance which folder contains the most space and even sort by size.

Note, though, that this will only show you the files in iCloud Drive; your space may be used up by other features like email or photos, if you enabled iCloud Photos.

I understand what you mean. The problem is that iCloud drive is not a good representation to understand what you have on iCloud unless you sync everything.

To understand what you have on icloud alltogether it's best to go to icloud.com

From here you can access the diferent folders that are created by the different applications and devices. The problem is that Apple does not provide sorting options or list views making it difficult to use. You either do manual or use icloud for Windows and do all the managing there. Granting you all kinds of additional options for sorting and managing that Apple on Mac or their website doesn't provide.