Find in your forumdisplay template:
<!-- / controls above thread list -->
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center" style="border-bottom-width:0px">Replace with:
<!-- / controls above thread list -->
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" height="40px" align="center" style="border-bottom-width:0px">Change the 40px in the height attribute to you desired setting.