If you have a smaller VMware infrastructure, you are probably running vCenter with its database on a SQL Express instance. This works fine, but eventually that database will fill up if you don't adjust the retention setting in vCenter, because SQL Express is limited to a maximum database size of 4GB.
This happened recently to a friend of mine, but in my haste to create a monitor in PRTG to monitor the size I neglected to get a screenshot of what that moment looks like. For that, dear reader, I apologize.
What I CAN tell you is how to find out how big yours is, and how to keep it from growing too large. The size of the database can be found by looking in C:\Program Files\Microsoft SQL Server\MSSQL10_50.VIM_SQLEXP\MSSQL\DATA\VIM_VCDB.mdf. The path may differ based on whether you have a 32 or 64-bit SQL installation, and what you named the file, but that's where mine is. That MDF file is your database (the LDF file with the same base name is your Transaction Log file, just FYI). So, you don't want this file to get near 4GB. I have a sensor in PRTG (my monitoring software) that keeps track of the file size, so I can look at trends or whatnot. It would also be fairly easy to throw something together using powershell to alert me if the file grows over a certain threshold.
What I would also do, however, is go into vCenter, click Home-->vCenter Server Settings and adjust the Database Retention Policy to something smaller. I believe the boxes are unchecked by default, so there is no retention policy, meaning that it saves everything forever. See this picture:
Your needs might be different - I have VeeamONE monitoring my stuff, so I elected for 30 days.