I thought I did a pretty exhaustive search, but I missed your first link. That's exactly what I was looking for! Based on your tests I'm thinking 100 grain capacity should give me a 100 yard range with steel balls. Especially since I can change the elevation of the mortar.
About the powder chamber design, from what I've read it seems like a long and narrow is more efficient and reliable than a short, squat one. Having the vent inside the powder column is probably the most important though.