Apple Mail is calculated dynamically by the software. If, as a prior user discovered, you see that you have 500MB of Mail, deleting or storing the Mail is you best option.
If, however, your Mail is around 50MB, there's no concern. The size will change for every user.
Now, Endoo, you mention that your email space continues to grow. Perhaps you could answer a few questions to pinpoint the issue.
1. What is the size of the email space? How much has it grown, and over what period of time?
2. How are you measuring the space?
3. Is Mail causing problems? Does it work properly? Specify the issues.
4. There seems to be an urgency to your question - "...need a solution fast." Is there a problem with your machine/Mail?
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