coal size vs size of coal

coal size and size of coal both are nouns.

Word	Noun	Adjective	Verb	Adverb
coal size	Yes	No	No	No
size of coal	Yes	No	No	No

As nouns, coal size and size of coal are synonyms defined as:

coal size and size of coal: the commercial size of coal, as broken and sorted by the coal braker.

coal size (noun)	size of coal (noun)
the commercial size of coal, as broken and sorted by the coal braker.	the commercial size of coal, as broken and sorted by the coal braker.