If you want to find out how much storage your SharePoint site collection is currently using then you can use the following REST url in your browser
://<Replace your Site Collection Url>/_api/site/Usage/Storage
You will get response similar to following. Please note SharePoint report storage in bytes so to get the size in MB you need to divide it by 1048576 .
In following example: storage in MB will be 8038715 / 1048576 = 7.67 MB