【For OneDrive app (Windows) users】

You can check the procedure from Microsoft's support page.

【For those checking with a browser】

1) Perform steps 1-5 described in page 2 of the leaflet, "[2] When starting using a browser," and log in from the following URL using a Web browser.
https://onedrive.live.com/about/en-in/signin/

2) From the Bento menu (9 dots aligned 3x3) button in the upper left corner of the screen, select "OneDrive" to switch to the home screen.

3) Click on the gear icon in the upper right corner of the screen > "OneDrive Settings".

4) On the screen that opens, click on "Site Settings" > "Storage Metrics".

<Reference>
Q:[Windows] Checking the capacity used
Q:[Ubuntu] Checking the capacity used