I have items which are in the bag as a stack, and every time I go to post, I have to make certain that I'm posting the right size stack (ie, one scope at a time, not the whole stack of them!). Every. Time.

It would be very nice to have something similar to the existing parameter
/aux post duration
such as
/aux default stacksize

The reason I haven't done this yet is that I'm not sure how to handle the behavior when you have less than your saved stack size. Currently the slider doesn't allow you to select a larger stack size than you could post and automatically reduces it, but that would be annoying if say you usually wanna post a 20 stack but then it would always save the size of the remainder at the end and not 20.

edit: actually I'm talking about persisting the stack size for each item but anyway I think that would be necessary because a generic setting seems rather useless as it obviously depends on the item what is better.

It seems to me that the slider should max out at what you've got - max(amount possessed, saved stack size). However, I don't see this concern as affecting the default stack size as requested because if the default stack size is lower than the default stack size, it shouldn't hit your issue above.