Exchange has a maximum database size of 16GB. You can temporarily increase this if you exceed it by editing the registry. To do so, open a registry editor (Start -> Run regedit) and browse to this location:
HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesMSExchangeIS
Now, find the name of the server whose database you would like to increase the size of and click on it. Then, click on either the folder that starts with Public- or Private- according to which you want to increase the size of. Now add a Reg_DWORD with a name of:
Database Size Limit in GB
Now set the setting for the limit to 17GB (just type in 17) and reboot the server.