This is to add to what Sally J said (so be sure to read her post first, if you haven't already done so).
Here is a guide (not hard-line specifications) about yarn, plies, sizes, etc.
< [
www.yarnforward.com] >
The above leads to a page about "tension" (or gauge), but be sure to follow the links about "wraps" (a method of measuring yarn size or thickness) and "yarn weights" as well to have a fuller understanding.
As Sally J mentioned, the most important aspect will be GAUGE, as this affects not only the size of the finished garment, but the "drape" (stiffness) of the fabric. ALWAYS, *especially* when substituting yarns, make a gauge swatch in the pattern stitch so you can be sure the results are what you expect and are pleased with.