Size of textareas for freetext or wiki CF

we are working with RT 3.4.1 on debian with mySQL. What can I do to enlarge the width/num cols for freetext or wikitext custom fields? I guess there is a html template to be cutomized, but I don´t have a clou where. I found the template ‘html/Elements/MessageBox’ with the following entry:
"$Width => $RT::MessageBoxWidth || 72
$Height => $RT::MessageBoxHeight || 15
$Wrap => $RT::MessageBoxWrap || ‘HARD’"
but nothing happens (well: nothing I see) when I change the number 72 and again I dont have an idea where this ‘MessageBoxWidth’ is found. Every hint is very welcom!

thanks a lot