Slatted floor spacing

I have taken Casey's advice and am going to redesign the flooring of my coop with a slatted floor, afterall it is a work in progress.

Simple question really, I take it the poop tray remains underneath the slats. But what sort of spacing should there be between the slats be? I have trawled around but havent found and info from the web. I dont want to put them in and find they are to far apart that the chickens injure themselves or to close together so the poop doesnt fall through. 

Thanks