In general you might say the Missy Fit is for US sizes 2-12 and the Plus Size is size 14-28. But every body is differently proportioned, it is not always such a clean difference. The Missy Fit is for bodies that are not so curvy, the Plus Size pattern is for people whose bodies are more curvy.
The DIY dress form patterns have 10% negative ease on some of the horizontal measurements - bust, underbust, waist. Do not try to wear your dress from to check the fit, it is not meant to fit you. (The pattern for the dress form cover DOES NOT have any negative ease)
If there is any problem with your pattern (measurements came out too big/too small, you input wrong measurements, you lost/gained weight, etc) we can reprocess the pattern for you. (We charge $2 for pattern reprocessing)
We would recommend making a muslin/test since it is so much work and frequently the pattern you use for the dress form is very special, so you don't want to waste your good fabric.
Sometimes the pattern comes out perfectly, and sometimes not. Every body is so unique and different, we are not yet able to have every pattern perfect yet. (YET!)