Hi TF12,
Yes … the more information you can provide them about you (body type, sleeping positions, preferences etc.) and mattresses you have tested or slept on in the past that you have done well with the more you will help them to help you make the best possible choice (see mattress firmness/comfort levels in post #2 here).
It would depend on your body type, the slat dimensions, and on the type of wood (see here) and on the overall construction of your bedframe because the goal is to end up with a support system that has minimal or no flex under your weight but in general with a polyfoam core I would suggest 5" as a maximum with standard 1x3 slats (which are actually 3/4" x 2 1/2") but 3" or less would be much better and more supportive yet.
Phoenix