There are standard Southern Wax Myrtles and semi dwarf types. This would explain the two general size ranges you mentioned. However the standard types generally are not going to get more than perhaps 15 or so feet in most cases. The semi dwarf types may stay even as low as 6-8 feet in height.
I think if you want something over 18 feet you should consider Carolina Cherry Laurel or perhaps a Little Gem Magnolia.
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