The TDM Portal log size is controlled by the %ProgramFiles%\CA\CA Test Data Manager Portal\conf\logback-tdm.xml file.
The logging is configuration is under the <!-- file appender --> tag, on line 24:
You can change the maxFileSize to meet your needs. You will need to stop and restart the CA Test Data Manager Portal service after making the change.
When you upgrade TDM Portal, you run the risk of the logback-tdm.xml file being overwritten, so you will need to check this configuration setting, and confirm that it is set custom file size, after each upgrade.