Browsers Firefox and XPiNC (XPages in Notes Client - that is XULrunner based on Firefox) show Dojo Text Area (dijit.form.Textarea) always with at least two rows even it contains only one line of text.
Other browsers like Chrome and IE work like expected.
You don't have a chance to change that behavior with style "height"/"minHeight" or parameter "rows".
The only solution I found is to create an own Textarea widget. But I am not sure if it's worth it...
This issue shows up only for contents with one line. As soon as you have two or more text lines Dojo Text Area's height adapts exactly - for all browsers.