Ask Mr. Smarty Plants

There is no upper limit to the size of tree that can be successfully transplanted. Cost will usually be the limiting factor for transplanting large trees. Large trees are traditionally measured in a unit called DBH, or Diameter at Breast Height where "diameter" (also referred to as "caliper") is measured in inches and "breast height" is universally defined as 54" above ground-level. The cost of a large tree will increase by roughly a factor of ten with each 1" increase in trunk caliper. However, other considerations may come into play in costing a transplanted tree: canopy spread, soil conditions at
both the dig-site and at the transplant-site, distance of travel, tree species, etc.

Keeping a newly transplanted tree alive can be tricky and even the best care may not result in success. Often, the act of digging and transporting, especially during hot, dry weather, stress the transplanted tree to the point that it cannot recover. Root and trunk wounds created during the transplanting process may create entry-points for disease pathogens which can kill or seriously damage a tree over time. Finally, the planting site itself may be inappropriate for the transplanted tree where, soil, shade, and water conditions are not compatible with the tree.

The single best decision a homeowner can make in going through this process is to choose a very experienced and highly-reputable tree company. They should work with a certified ISA (International Society of Arboriculture) professional in selecting the appropriate species for their landscape and in selecting the specific tree to be transplanted. The homeowner should follow their arborist's recommendations to the letter in caring for their tree, especially during the first two years after transplanting.

Finally, here is an excellent article, Newly Planted Trees, from Clemson Extension, the South Carolina Forestry Commission, and the U. S. Forest Service that you might find useful.