When figuring crane boom length on any project, keep in mind the radius from where the crane will be placed.
The boom can be raised at an angle, and a hoist with a hook can be lowered to the ground. This allows the crane to pick up objects nearer than the radius of the fully extended boom. But the crane can never extend beyond that radius. Each crane you consider purchasing or renting will have a printed table that specifies its load weights and lifting radius.
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.