Mac Mail Limits (messages and mailbox size) We use Google Apps and with users approaching 250K email messages and over 25GB of storage Mail App is getting ever slower for most users. 

Are there any guidelines on how large we should let the mailboxes grow? 

I have not been able to find any information on limits (even from user experience) online.
The Limit is not something programmed into Mail.

In effect, your Hard-Drive storage and the Performance of your Mac are the only limiting factors, not some count of messages in terms of size or number.
As you already noticed, the app gets really slow with Mails over 100k or even 200k on current hardware (perhaps except the Mac Pro).

It is not recommended having such a big MailBox at all.

How I think it should/could be handled :


Don't go beyond 10k Mails (in your main Postbox)
Move Old Mails into a Mail-Archive


Having your own Server for that helps a lot.
I use a OS X (Mail-) Server  with a Roundcube Frontend and a MySQL Database.
In our company we have a Linux Mail-Server and a Roundcube Frontend with a MySQL DB.
The Server can handle those Mails a lot better than Apple Mail. Also roundcube as a Frontend is much faster when going through severel houndred of thousands of mails.

If you're reaching a Mail-Count of more than 200k, you are probably collecting a decade of mails (unless you are writing a lot).
After 3-5 Years you should store those mails on a Backup Server (Maybe at a different location), safe for nobody to access and safe regarding any kind of loss.