size of coal vs coal size

size of coal and coal size both are nouns.

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

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

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

size of coal (noun)	coal size (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.