Relatively speaking, 13 GB is not large at all when it comes to common Exchange Server database sizes. The answer to your question depends on a number of factors, such as whether you intend to download information from other sources into the Exchange databaes, or simply allow users to retrieve their messages from the Exchange server using POP -- or both.

If you intend to download messages from other sources using POP then the size of your Exchange databases will likely grow.

If you intend to allow users to retrieve their messages from the Exchange server using POP and messages are retained on the server after download then the size of your Exchange databases will likely grow. On the other hand, if messages are deleted from the server after download then the Exchange databases will likely shrink in size. This, however, depends on online maintenance completing, so that white space can be recovered.

You can speed up the white space recovery process by performing regular offline maintenance (defrag). Of course, this comes at the cost of the Exchange server not being available while maintenance is being performed.

Generally speaking, whether the Exchange databases grow or shrink in size depends on the ratio between the incoming and deleted messages; also the size of those messages and their attachments; however, the natural order of things is for the databases to grow over time. To minimize this, you can perform regular database pruning by expiring or deleting items that meet a certain age criteria.

I don't know what kind of organization this is for, but before enabling POP, you might consider if it presents any legal issues for you and your organization. You should consult HR and legal counsel before deleting or purging any data.