To be fair, the shoutbox is for idle banter. Anything that's an important discussion SHOULD be in a thread.
Some things can still be semi-important in the shoutbox and so it'd be nice to rewind a little bit. A day, or even a few hours should suffice, but a few dozens of posts like right now seems too small.
It's never going to happen as you might expect it, unfortunately. I've set the full archive to 150 messages - it was on 75. That slows the page load down by a second or so. Any longer and it gets considerably slower.