Right, reducing the length of the page for the long messages in postbit & postbit_legacy. In your forum, many members have written long message (maybe up to 10,000 characters), made that webpage is longer, more viewers will be offended because they have to slide the screen to see it. You can limit the number of characters, however, why do you do that ? Let your members write unlimited words. And, you do not have to worry about the length of the webpage.
Additional, you can reduce space between message content and signature !

• First Step : add new css in additional.css (Edit Templates -> CSS Templates).
  Paste following code into bottom of additional.css template (or anywhere in that template) :
  
  Code:

  ---

  /* Make a shorter message in postbit_legacy */
.g_short {
    min-height: auto !important;
    max-height: 350px; /* Custom for your */
    overflow-x: hidden;
    overflow-y: auto;
    padding-right: 10px;
    text-align: justify;
    margin-bottom: 0 !important;
}

/* move up signature */
.postbitlegacy .after_content {
    clear: both !important;
}

  ---

• Second Step : edit in postbit.css (Edit Templates -> CSS Templates) :
  Find this code :
  
  Code:

  ---

  /*post bit*/
    ...
    and
    ...
.postbitlegacy .postrow.has_after_content {
    padding-bottom: 4em;
}

  ---

  Add below and edit code :
  
  Code:

  ---

  /*post bit*/
.postbitlegacy .after_content {
    clear: right !important;
}
    ...
    and
    ...
.postbitlegacy .postrow.has_after_content {
    padding-bottom: 0em !important;
}

  ---

• Third Step : edit in postbit & postbit_legacy template (Edit Templates -> Postbit Templates) :
  The code you will do all same in these templates. Find this code :
  
  PHP Code:

  ---

  <div id="post_message_{vb:raw post.postid}">
    <blockquote class="postcontent restore ">
        {vb:raw post.message}
    </blockquote>
</div> 


  ---

  Make it same ...
  
  PHP Code:

  ---

  <div id="post_message_{vb:raw post.postid}">
    <blockquote class="postcontent restore g_short">
        {vb:raw post.message}
    </blockquote>
</div> 


  ---