I was iffy on the size and went with the smaller of the two choices 18h and 21.
21 would be ideal as a seat for those who have a hard time getting up from a chair. But the smaller 18, is a better height if you want a chair and stool to put your foot up  on to soaplather... and for the ladies to put their foot up on to shave their legs. Etc..
Assembly easy, care simple... Look and function fantastic.