I want to use 18 x 18 inch tiles on the walls in the bath but I'm afraid the tiles are too heavy and will slip. Is this something that I should be concern with?

18" tiles are fine for wall applications. Just use a quality modified thinset and use some spacers/wedges to keep your tiles steady and even.

I've installed 24" tile that was 1/2" thick on walls before.

You'll be fine.

Good luck

Matt
tile contractor
Charlotte, NC