Decide where the crane should be placed. Plan to locate it on a sturdy and flat space, with no obstructions for its boom. Look for any overhead obstructions like tree branches or power lines.
Measure the distance from the crane location to the objects you will be moving, as well as the place you will be moving them to. Estimate the weight of the objects you will be moving.
Find the appropriate weight and lifting length on the crane load tables, using the longest length and heaviest load requirement you might need for the entire job. If you are renting or buying a crane, there should be several different tables to look through.
If the radius of the reach you need is between two of the values on the table, use the longer length. Also err in favor of a heavier weight, as over-taxing the crane can cause it to fall over.