Documentation for JIRA 5.1.x. Documentation for other versions of JIRA is available too.

Unable to render {include} The included page could not be found.

To work around the fixed size of a comment field, edit <jira-install>/atlassian-jira/WEB-INF/classes/templates/plugins/fields/edit/edit-textarea.vm.

To change the size for all rows:
  
$!rendererParams.put("rows", "2")
To change the size for a particular custom field:

In this example, the custom field's id is 10220. You can get this value from editing the custom field and checking the URL.

#controlHeader ($action $customField.id $customField.name $fieldLayoutItem.required $displayParameters.noHeader)

#if ($!customField.id=="customfield_10220")
  ## setup some additional parameters
  $!rendererParams.put("rows", "2")
  $!rendererParams.put("cols", "40")
  $!rendererParams.put("wrap", "virtual")

  ## let the renderer display the edit component
  $rendererDescriptor.getEditVM($!value, $!issue.key, $!fieldLayoutItem.rendererType, $!customField.id, $!customField.name, $rendererParams, false)

#elseif ($!customField.isRenderable() && $rendererDescriptor)

  ## setup some additional parameters
  $!rendererParams.put("rows", "2")
  $!rendererParams.put("cols", "40")
  $!rendererParams.put("wrap", "virtual")

  ## let the renderer display the edit component
  $rendererDescriptor.getEditVM($!value, $!issue.key, $!fieldLayoutItem.rendererType, $!customField.id, $!customField.name, $rendererParams, false)

#else

  <textarea name="$customField.id"
            id="$customField.id"
            class="textfield"
            rows="4" cols="40" wrap="virtual"
  >$textutils.htmlEncode($!value)</textarea>
#end

#controlFooter ($action $fieldLayoutItem.fieldDescription $displayParameters.noHeader)

There is a feature request to allow this cusomtisation from within JIRA at JRA-20248.