To increase the email size limit in Plesk, you can follow these steps:

Step 1: Log in to your Plesk control panel using your administrator credentials.

Step 2: Once logged in, navigate to the "Mail" tab or section. This section is typically located in the left-hand sidebar or under the "Domains" category.

Step 3: In the Mail section, you should see a list of domains or email addresses. Locate the domain or email account for which you want to increase the size limit and click on it.

Step 4: Inside the domain or email account settings, look for an option related to mailbox settings or mailbox size limit. The specific wording may vary depending on the version of Plesk you are using.

Step 5: Adjust the size limit according to your preference or requirements. You can either set a specific size limit (e.g., 500MB) or choose an option like "Unlimited" if available.

Step 6: Save the changes or apply the new settings.

Step 7: Repeat these steps for any other domains or email accounts that require a size limit increase.

It's important to note that some hosting providers may have overall disk space limitations that could affect the maximum email size you can set. If you encounter any issues or limitations beyond your control, it's recommended to reach out to your hosting provider's support team for further assistance.