We install Add2Exchange with SQL Express 2005 (up until 2011) which had a 4 gig size limit.  This limit is rarely reached but on occasion it is.  You can easily upgrade to SQL Express 2008 R2 which has a 10 gig limit, which is usually more than sufficient. 

To identify the problem, you will receive these error messages

In the Add2Exchange Event log you will receive:

Error copying and transaction rollboacks

You will also receive an Error in the Application log such as this:

CREATE DATABASE or ALTER DATABASE failed because the resulting cumulative database size would exceed your licensed limit of 4096 MB per database.

Resolution

Upgrade to SQL Express 2008 R2 with 10 gig limit - Free

or Read this article:

Procedure

Stop the Add2Exchange Service

Run the installer which gives you the option of upgrading your existing instance. Select A2ESQLServer to upgrade.

SQLEXPRWT_x64_ENU.exe
SQLEXPRWT_x86_ENU.exe

http://www.microsoft.com/en-us/sqlserver/editions/2012-editions/express.aspx

this post outlines the straightforward process. No reboot is necessary.

http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=200

You have to do the 32 bit upgrade, even on a 64 bit system, because from this matrix it would appear as though you cannot upgrade 32bit to 64bit, at least not directly.

At the writing of this article, you have to install the SQL2008 R2 upgrade on the same replication Server where Add2Exchange is installed.  In the future, you will be able to connect to a new ore existing full edition of SQL Server on another box and communicate by TCP/IP.

You can also upgrade to full SQL on the same box (at present) for a fee to virtually avoid limits all together.