How to restrict the Email’s based on size?

As an administrator, you can create rules that specify how to handle messages that meet the criteria that you define. Conditions can be based on file type, file name, and message size.

For example, the users want to restrict the Email which is more than a 25MB due to storage limit so you can use an attachment compliance feature to restrict the Emails 

Restrict the Email’s based on the size

1) Log in to the admin console (admin.google.com)

2) Go to apps - Google workspace - Gmail

3) Go to Compliance - Attachment compliance - Add another rule

4) Enter the short description of the rule - Select inbound and internal receiving 

5) Select Add in the second option

6)Select File type - Message size and Give the maximum size of the Email

7) Select Reject message and give the message you want to show when the Email size exceeds.

8) Save the changes.