Two solutions :
1) use a RichText widget for my TV, and just tweak the height from the properties
2) use CSS to assign a different height to the textarea
2nd one is the preferred method
Edit : I just realized that you might be speaking of the manager’s textareas and not the frontend...
Well, that’s a different story you’ll have to hack into the manager theme then, and maybe into the js I never toyed with that can’t give you a pointer without digging into it...
Other (simpler solution) is to install the
Resizeable form fields FF extension (or buy a Mac : Safari has this built in, but that’s not cheap

)