The GUI lets you pick manually which one to choose, but there is no suggestion of which one has least users on it.
However, this is easily scripted in PowerShell:
get-mailboxdatabase -server myserver | get-mailbox | group Database
This will show you which mailboxdatabase has what number of users. You can expand this to ALL your org by simply omitting the -server switch:
get-mailboxdatabase | get-mailbox | group Database
Thanks, ~vivek