Since you already know the sq footage, you really do have the dimensions, but here is what I think you may be after:
If you are planning to mulch for example, take the length x width (in feet) x Depth I
f you are planning to have 3" of mulch do LxWx .25 or for say, 4" of mulch LxWx .33
Now you have the cubic feet. Take cubic feet and divide by 27 for cubic yards needed (of mulch or peat moss or whatever! Good luck!
Ellie Shorb, Realtor
Long & Foster, DC, MD, VA