[tbworld]

To assist you in CSS, a link to your board is almost necessary.

You can give this a try, it may not be exactly what you are looking for. Possibly, a better solution might be altering the template for finer control; as the change posted below will alter all line heights in the 'last post' block.

Past this in the bottom of your 'additional.css' template.

#forums a.username {
    line-height: 20px;  /* Adjust this accordingly */
}