if you've designed the frame with a ridge that brings loads down to the ground via a post, you can go with nearly any knee wall height you want. although, in some cases, going tall would have to be looked at in terms of wind loading.

if you are relying on the posts to take outward thrust, then the tie / post joint needs to be designed for this - and the post needs to be thick enough to not snap off above the tie when you load the roof.

for purlins, 4', 3', 2', 6', 4' 3 5/16" on center etc are all acceptable, really depends on what you plan on doing above - SIPs, conventional framing, just sheathing?

size the purlins based on the tributary area, then size the rafters based on their area, and take into account material removed to house / join the purlins, and size up accordingly.

you can start to chase your tail... but that is usually the process.