A gravel or crushed-stone pad is one of the best ways to go for your shed. It is not needed, but here are some considerations. 

Sheds placed on gravel (or crushed stone) platforms will have the advantage of:

• better drainage

• more level surface 

• less settling of the shed over time

  fine aesthetics (a sign of intention & beauty)

If you do plan to set a Crushed-stone base, take note below:

• gravel base would extend about 1’ out on all sides  (to 10'x12’, for example, for an 8’x10’ shed)

• It will not take a lot of gravel if the ground is close to level [3” to 4” thick is best]

• Staying with the 8’x10’ shed, 80 sq ft  at 4” thick will be almost exactly 1 yard of gravel (1 yd = 27 cu ft) (cu ft = ft³ = cubic feet)

Regarding stone size:

Stone diameter about 1 1/4” to 1 1/2” should work well

OR

Following is a link for information on a type of stone -called “item #4”:

http://www.lawtonadams.com/index.php/item-4-a-base-material

Item #4 contains finer material as well as the coarse stone, meaning it will eventually pack well—though not necessary for the firewood shed -loose stone is okay

I would check with local landscape companies for leveling and installing a gravel pad