You can create a file in the conf directory named userstyle.css. In that file you can set css values. For the default dokuwiki template you can set the wrapper div to a specified height. That will control the dokuwiki page length. For instance:
div.wrapper{
height: 600px; overflow:auto;
}
You have to make sure that the overflow doesn't cut off any text. However, you also have to understand that this might give you intrusive scrollbars around the text.
A little neater alternative is to set the entire body height to a fixed length:
div.wrapper{
height: 800px; overflow:auto;
}
With this you might avoid some horizontal scrollbars.