Header, help.. Options

[Ricky Himself]

Hi, Im trying to make my header, the "enter my word" thing the same size as the xanga advertisement above it. Width that is. When I do it, it leaves like a black line under it..its wierd. I made it a little bit smaller so u can see. Look to the bottom right see that black line sticking out? Well it only gets longer as I make it smaller...what is it and how can I get rid of it? Thanks.