hello Sandy ,
not sure of position or size wanted but here is an idea.
make the front higher than the tallest person likely to get logs from the store .., nothing worse than cracking open your head whilst collecting logs in the dark. The back only needs to be say 6" higher than the front to allow good rain/snow fall.
The corner posts can be just logs or fence posts dependent on what you have . Knock them in the ground if you want it permanent .Good solid 3"-6" ones work best , they are sturdy , last a long time and don't fall apart the first year.
Why use plastic for the roof ? if you have it there already fine , but otherwise I would go for corrugated iron or even timber planks. Sides only need to be slats really , that way it allows plenty of air in to season the wood and to help prevent mould.
If the wall you are using as the back , is your house wall , make sure you don't bypass the damp course .
An area/store 8 feet x 6 feet x 4 feet will hold a chord and a half of wood .
cheers
Russ