"Do you have to have a certain number of posts for it to be bigger?"
No. O_o;;
Okay, enter the original banner into imageshack, press it again when it uploads to that full size and get the direct link. And make sure when you are uploading on imageshack, you haven't clicked the restricted sizes. So, copy and paste that direct link into the "Insert Image" tag. (The mountain, hover over to read it) And it should work fine.