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:
<maxFileSize>100MB</maxFileSize>
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.
Note: 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.