Hi sleeping,
Outside of Sleepings great suggestions (and you can see an example here of multiple center beams) … a good guideline is that the surface area of the slats is at least 50% or more of the total surface area. This means that I would tend to use spacing with 1x3 slats (which are actually 3/4" x 2.5") in the range of 2.5" or less. Of course 1x6 slats would be stronger yet.
Closer spacing in the middle third of the mattress which carries more of the weight can also increase the support strength of the bed.
The type of wood used for the slats and its bending strength and stiffness can also make a difference.
Phoenix