I don't go with plans - I just figure it out on paper
I do everything 16" on center, 2x4 Studs are 16" on center
Floor joists & rafters the same
What are you going to store in the shed? A garden tractor?
Are you going too build a wooden floor or cement?
I went with cement as existing shed floor was cement ( I added 10x10 on)
I also went with double doors 5' wide so I can get larger items in & out easily. 8x12 isn't that big, do you have snow load where you are?
A 2x6 rafter will span 11' 16" on center
A 2x4 will span 7' , but I never use 2x4 for roof rafters - snow load
I use plywood, not the OSB - just my preference
And my shed is a pool cabana - so there is a filter in there & can be subject to water with floats etc being stored