Using advanced search

https://support.google.com/mail/answer/7190?hl=en

To see how much of you 15 GB Google Quota is being is being used by Gmail and other Google Services:

https://one.google.com/storage

In Google Drive see a list of files from largest to smallest

https://drive.google.com/#quota

How GMail stores your mail

http://gmail-miscellany.blogspot.com/2012/10/how-gmail-stores-your-mail.html

Gmail support center

https://support.google.com/mail/#topic=7065107

Why after you delete older emails for All Mail do they still appear after a search like before:2016/01/01