attempting the same thing, i was able to track the field size down to a CSS stylesheet inside workflow designer on our servicedesk server that controlled the description box size
we manually changed the text field size, which worked for about 15mins, but then it reset itself
if anyone else can find a way heres the location:
\\{server}\c$\Program Files (x86)\Altiris\Workflow Designer\Ensemble\App_Themes\Symantec\Layout.css
i don't understand css code but our web guy says look for WP_Middle an insert something like:
line.height = {number}
i would think it would be some sort of modification to the project theme, and reload project to server, but i haven't attempted it