iCloud & iCloud Photos · Storage Studio and iCloud
How can I see how much space iCloud is using on my Mac?
Short answer: Open Storage Profile from the More menu (•••): it measures iCloud and other synced folders alongside your Mac and external drive. Preferences → Rules also shows how much disk is caching cloud files, per provider.
Open Storage Profile from the More menu (•••). Its Where it lives and Connected cloud accounts sections show how your storage splits between this Mac, your external drive and the cloud, and iCloud and other synced folders are measured there too.
Preferences → Rules → Free Up Space includes also shows how much of your internal disk is currently caching cloud files, per provider.
macOS's own view is in System Settings → General → Storage, which lists iCloud Drive and Photos among the categories. Keep in mind that online-only files take no real space on your Mac, so they don't count toward what iCloud is using locally.