That limitation to the textbox was removed in CVI 6.0. We no longer limit the number of lines or scrolling capability in the text box. We mistakenly did not fix this in the documentation in CVI 6.0, but have fixed for the next release. If you are using CVI 6.0 though, that limitation is no longer there. If you are using a version older than 6.0, then there isn't really a workaround with the textbox without upgrading which you can do at http://www.ni.com/upgrade.
Best Regards,
Chris Matthews
National Instruments