Brian,
For a fixed dimensions multiple-line-textfield, you can use the maxChars property to control the maximum number of character in the textfield. Unfortunately, the property such as maximum number of lines does not exist. Otherwise, this property would be a better choice for you.
However,you can write some javascript to count the number of lines of the text that was entered in the textfield and alert your user that his/her text entered might not be printed properly.