This occurs more often than not because the size of the Model Database on the SQL server instance has become larger than expected
All newly created databases use the Model database as a 'template', and will start out no smaller than the Model database.
For some odd reason the ESB Toolkit Configuration Utility seems to rely on a powershell script that is embedded in an .exe that sets the size of the DB with a hard-coded value (awesome)
Now that the science is over, the fix is to shrink the model database on the SQL instance that you are attempting to attach the ESB toolkit to.
Simply Right Click the “Model” database from the object explorer in SQL management studio (pictured above), hover over Tasks, from the resulting menu select “Shrink” then finally … click “Database”
Now click the “OK” button at the bottom right of the window (pictured above) and then SQL will perform the shrink on the database
Once this has completed, click the “Apply Configuration” button again at the top left of the ESB Configuration Tool window (pictured above) and your Configuration should complete successfully