That's the pattern industry's dirty little secret: you DON'T just cut one size and sew it and pray it fits.
It does say somewhere on the pattern that they're "multi-sized for perfect fit!" or some similar bullshit. It basically means you choose the size-line that fits your numerical measurements at crucial points, like chest-waist-hips, and if they happen to be two different sizes, you just draw a line from one to the next to connect the size 12 bust with the size 16 waist. It's something we all end up learning to do anyway but it amuses me that the pattern right out says so and yet a lot of people never see it. It's usually printed on the pattern tissue, not in the instructions.
Depending on how you need the garment to look and how well it needs to fit, you most likely will need to not just do this, but also check other measurements - the width across the back across your shoulders, the length of the sleeve, the length of hip/rise and the length of leg on pants, etc etc etc. Everything is standardized to the least common denominator, so if you just close your eyes and cut a pattern the same size all around, you could end up with something that not only doesn't fit you, it doesn't fit in ways that can't be solved by merely taking up seams. You would have to start from scratch and recut the whole thing.
When in doubt, severely in doubt, make a mockup and do all your fixings to that. But in a pinch, or if you only want to check a couple of things, you can use the pattern paper itself as your mockup. Gently pin or tape it to form the seams and slide it on yourself, and look in the mirror to check and see if everything sits in the right spot. Shoulder seam too low? Lapel folding too high? Sleeve totally way too big? You can make adjustments before you even lay scissors to fabric.
At the very least measure the pattern pieces as they're laid out, and look all over the pattern (usually printed somewhere on the tissue paper but sometimes on the envelope itself) for "finished garment measurements" listing. It might say that a size 16 garment which should fit a 38" chest is actually 42" at the chest, and if you're not supposed to have that much extra give in the garment, you'll have a hint that the pattern is not going to play nice with you.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com