What percentage of Chinese words are 1-character ? ... 2-character?
an unscientific approach: the 國語辭典
, of taiwan, has about 164489 entries, using grep + wildcards, one may find:

12540⋯7.62% are 1 character entry (^.$) e.g. 八, 扒
87516⋯53.20% are 2 characters entry (^..$) e.g. 八伯, 八拜
21836⋯13.28% are 3 characters entry (^...$) e.g. 八拜交, 八敗命
35781⋯21.75% are 4 characters entry (^....$) e.g. 八百羅漢, 八百壯士
2562⋯1.56% are 5 characters entry (^.....$) e.g. 八分休止符, 八風吹不倒


how to define / interpret "word" in the finding, well, it's beyond my knowledge :(