Exchange Server 2010 has virtually no limits on the sizing of the Mailbox Databases, the theoretical size limit of a Mailbox Database is 16TB which is, of course, beyond every practical limit you can think of.
The Microsoft recommendation is to use a maximum of 2TB per Mailbox Database, if you have configured a minimum of three copies of the Mailbox Database in a Database Availability Group (DAG). When a DAG is not available, but you're using a solid hardware RAID solution then I normally recommend not to grow beyond 100GB per database. This is an acceptable limit when it comes to recovery scenario's.
However, the Exchange 2010 Standard Edition is practally limited to 1TB Mailbox Database, but this is enforced by a Registry key. you can find this key here:
The GUID of a particular Mailbox Database can be retrieved using the following Management Shell command:
Get-MailboxDatabase -Identity "" | FT Name, GUID
Change the "Database Size Limit in GB" DWORD value into the desired size.
Please note that this is only an acceptable configuration if you're using multiple copies of the Mailbox Database!