Problem solved !
I had replaced the logo_phpBB.gif with my own logo which is much wider than the standard logo. The FAQ,Search,Memberlist,Usergroups bar seems to be a fixed width, so my logo next to that bar made it a certain minimum width. I just removed my logo and it's now behaving as I wanted it to.
Now to figure out how to integrate it with the rest of my site so it doesn't open in it's own window.
Thanks for the help, much appreciated !