Hi!

Our users are reaching 20 GB of their mailbox size. May we know what is the recommended mailbox size in Exchange 2013?

Thanks.

Hi 

There is no such in recommendation of giving a standard best practice for mailbox size as it varies according to every org structure based on Storage , Email  flow per day , Server hardware , memory etc..,

The better way i would suggest is to first collect the daily, weekly and monthly email statistics for the whole environment 

These scripts will help you on collecting the statistics 

Emails sent per hour - https://gallery.technet.microsoft.com/office/ec7940c1-be9a-45d5-9ea1-921677fa081c

Emails sent per day and size    https://gallery.technet.microsoft.com/office/f2af711e-defd-476d-896e-8053aa964bc5/view/Discussions

Emails sent per week  and size -  https://gallery.technet.microsoft.com/office/Total-Emails-Sent-and-c1daf5e7

Calculate the average size of the emails sent per user on daily basis from the above output

Based on that calculate the quota that you need to set for all the users 

Better you can set some  exclusions for VIP users

Also it would be good if you implement some archiving solution in place for emails older than 6 months so that your primary mailbox will not be loaded always

Hi ,

I agree with sathish suggestions and also in addition to that please have a look in to the below mentioned link.

https://social.technet.microsoft.com/Forums/office/en-US/052915dd-f081-4afd-b0ce-060225bc0f72/exchange-2013-mailbox-size-limits?forum=exchangesvradmin

On the above link we came to know about the maximum size that we can keep for a single mailbox in exchange 2013.

Adding few more points 

You need to make sure that all the mailboxes are balanced equally on existing mailbox servers before planning.
Measure the currently allocated disk size in each mailbox server based on the current value and expected value in 6 months
Calculate the growth of expected users in 6 months 
Calculate the current log space , database space and the expected value after 6 months
Always keep a buffer value of 25 percent on the total value of the disk space required to be on the safer side

Good Luck !!