If I'm understanding the question correctly, then the solution is quite simple. If I create a layout table, then create a repeating table inside of it, the repeating table will be limited to the width fo the layout table.
If your concern is with a text box, you can change the fields' Width to "Auto" (in the Size tab). This will increase the width of the field as your users input data.
If this doesn't answer the question, can you attach a sample of what you're trying to do?
Thanks,