I had a quick look at your board and although my screen resolution is only 600x800, I did not need to scroll when using the posting screen.
I believe the posting area (textarea) is a fixed width, so I don't think you could change it anyway.
If you feel the page is a little overcrowded, you could have the emoticons in a drop-down menu box instead of displaying them all. The only problem with that is that your posters might not recognise them all!
You can make that change in Board Settings >> Edit Board Settings.
Try that and see if it helps, or possible Chris could suggest something else, when he comes on later.