Header image size, Main header menu, secondary header menu, drop-downs

Hello! I was so thrilled to find a theme that met my pretty reasonable needs for my blog, www.agoodcourage.com. There's really only a couple issues I'm having. They should be easy fixes, but I'm a WP beginner and I don't want to break it. If someone can tell me what to do, I can do it! So, a minor issue is that I'd like to be able to adjust the required/suggested size of my header image to make it a little longer on the Y axis. If that's not doable, I'll live. Second, I would love to get rid of the main header menu (the white bar at the top that follows as you scroll) altogether. Tried to remove it myself, had to log into my CPanel and reset the whole shebungo. Third, I'd like to use the lower menu (secondary menu) alone. So far that's okay, but I lose the drop-down functionality when I do so. I see the theme preview here has drop-down functionality in its secondary menu. What am I missing there? Any help at all is greatly appreciated! I would love to stick to Adventurous but I need to be able to tweak those menus!