Hi Don
sorry for the very slow response - you may have experimented and moved on by now, but I wanted to answer.
We've just published a rather detailed document of best practices for Lidar at this link Image Management.. It includes a discussion of deciding sample size.
For a quick answer, I typically recommend you make your bare earth DTM no finer than 4 points per cell (or 2x point spacing), e.g. if point SPACING is 1.3 meters, that would be cell size ~ 2.6 meters (thus 4 samples in your DTM cell looking at 2 dimensions). However, your *average* point spacing will not account for areas with higher and lower point density - so in heavy canopy your spacing will be more coarse, and in truly bare areas your spacing will be better than the average value. I recommend using the GP Tool "Point File Statistics as Raster" and summarize using PULSE_COUNT to get a raster map of your outgoing pulse density, and then using POINT_COUNT to get a raster showing point density - then you can see areas that might have thin coverage. (one other detail - I said "density" above but note if you summarize this raster at 2.6 meter cell size, you'll get pulses or points per [2.6]^2 square meter - so if you want an actual numeric measure of density per square meter you have to divide each value by 6.76 to normalize)
I hope that helps!
Cody B
...oops, I didn't scroll down so I didn't realize Jake Skinner had already answered... Thanks Jake. I also didn't answer re: DSM; that's up to you but many users go as fine as 1 LAS point per cell, just to see the maximum structure - but it'll be noisy, so I usually make my DSM and DTM the same resolution just for consistency.