For the main reply box:
Sources/Post.php
Find
// Now create the editor.
$editorOptions = array(
'id' => 'message',
'value' => $context['message'],
'labels' => array(
'post_button' => $context['submit_label'],
),
// We do XML preview here.
'preview_type' => 2,
);
Replace
// Now create the editor.
$editorOptions = array(
'id' => 'message',
'value' => $context['message'],
'labels' => array(
'post_button' => $context['submit_label'],
),
'columns' => 60,
'rows' => 12,
// We do XML preview here.
'preview_type' => 2,
);
Change the values for columns and rows to your liking.
For the quick reply, you'll need to edit Display.template.php:
Find
<textarea cols="75" rows="7" style="width: 95%; height: 100px;" name="message" tabindex="1"></textarea><br />