There are two ways to prevent users from committing with an empty log message. One is specific to TortoiseSVN, the other works for all Subversion clients, but requires access to the server directly.
If you have direct access to the repository server, you can install a pre-commit hook script which rejects all commits with an empty or too short log message.
In the repository folder on the server, there's a sub-folder hooks which contains some example hook scripts you can use. The file pre-commit.tmpl contains a sample script which will reject commits if no log message is supplied, or the message is too short. The file also contains comments on how to install/use this script. Just follow the instructions in that file.
This method is the recommended way if your users also use other Subversion clients than TortoiseSVN. The drawback is that the commit is rejected by the server and therefore users will get an error message. The client can't know before the commit that it will be rejected. If you want to make TortoiseSVN have the button disabled until the log message is long enough then please use the method described below.
TortoiseSVN uses properties to control some of its features. One of those properties is the tsvn:logminsize property.
If you set that property on a folder, then TortoiseSVN will disable the button in all commit dialogs until the user has entered a log message with at least the length specified in the property.
For detailed information on those project properties, please refer to “Configurações do Projeto”.