I am 5'6" and, unless you have a extreme body geometry, an 18" is too big. I have a 17" cross country bike; but it is really suited for a single purpose (non-technical long distance single track). A 52 or a 16" are going to be a better fit for normal, utility, riding (for most single track I actually use a 15").

For my utility bike I got a "S/XS" and I still think it is too big. I have real trouble getting bikes that fit and I can assure you that a bike that doesn't fit is a bad idea. It is something that will push you away from riding.