with most sizing schemes, an 18" frame would be a medium. Though it depends what sizes are offered. Some makers have 17" and 19" instead. And some makers have strange sizing. I'm a medium sized person, right on the crest of the male sizing bell curve, and I fit an 18" fine. But my size in a Ritchey is 19"; in a Klein, 20"; in a Yeti hardtail, 17.5".
Yep, just what bulC said. You should also be looking at the effective top tube length to make sure it is not too long for you. If you don't know what a good length is then try measuring the effective top tube on a bike that fits you well. If you're completely confused by sizing then go to the Colorado Cyclist website and click on the fitting guide. It is an article that outlines concerns for frame sizing.
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