Thank you for the post.
I am able to replicate this with FileMaker Pro 12.0v4 on Mac OS X 10.8. Windows 7 displays more of the text than Mac OS X; however, the remaining text eventually cuts off after the contents exceed the available screen real estate.
I forwarded a full report to Testing and Development for review.
As a workaround, when using more complex dialog windows, use the New Window script step and create a new layout to go to instead.
This report has been linked to an entry in the Known Bugs List. Please post any comments, questions or corrections about that entry here or in a new thread. Please do not post such feed back to the Bugs List Thread.
Further to my original post, and as suggested by PhilModJunk, this is a link to the FileMaker 12 file that I prepared which demonstrates this issue.