Block size issue

Hi, I am pretty new to drupal and I'm trying to figure out how to edit block sizes. I've used wordpress in the past and all of that stuff was typically in either css or in a sidebar.php file.

Basically what I am trying to do is on the right i would like a 336x280 block and underneath it 2 columns. 150 in width each. Any idea how I can go about doing it? In that 336 box, I am going to place my ad code and I don't want it overlapping the block as it looks goofy.

If you go to www.juvnenilehumor.com/drupal you will probably get a better idea of what I am talking about.

Any help would be much appreciated. I can be contacted at ctulumba@gmail.com

Thanks!!!!!!