Answer: For a professionally done plan, this would depend on the conventions used by your office. In addition to the drawing you would have a specifications sheet to indicate the specific height and caliper and grade of the tree to be supplied.
If it is a plan for yourself, you can use a size that makes sense to you. The tree will grow over time so its size at planting is not the most important. You need to keep in mind the mature spread of the tree (check the cultivar you are using) besides the size when it is planted.
On a plan, you would draw those three at the same size if you simply want to indicate a large tree is going in. (Trees are larger than shrubs, deciduous will have a smooth outline, evergreen a jagged outline.) It is more important to accurately indicate the exact location for the rootball rather than worry about the canopy size at planting.
I hope this helps.
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