Understanding ease is one of the most difficult concepts for garment knitters to truly understand. Knitters often show me gorgeous work and then tell me how very unhappy with the result. When I ask questions, their disappointment most often comes down to the gap between the garment that they imagined and the one they knit.
Often the gap has to do with the knitter's personal ease preference. This preference is a moving target, as we age, gain or loose weight and adapt to changing silhouettes in mainstream fashion, our preferences are constantly shifting.
Jared has an excellent post here with photos of two different models in the same garment. Each model has slightly different measurements but the one garment fits each model. The slight difference in their measurements moves the garments ease from in the case of the Reine Cardigan from one inch negative ease to two inches of positive ease.
How can you reenact this helpful experiment for yourself without knitting the same thing twice? Get out your measuring tape, head down to a store and start trying on the same sweater in three different sizes. Try on, compare and measure one that is smaller than you normally would wear, one that you consider to fit well and and one that is too loose. Don't just measure at the bust-line, check as many measurements as you can. Pay attention to the fact that you will have different ease preference for different parts of your body which means that the best set of measurements may come from all three garments not just one. I like less ease on my sleeves than most patterns include, and I want more around my torso. I hate an overly long sleeve but know many knitters who love a sleeve that is well below their wrist bone. Once you have this information collected it will help you to choose the correct size and recognize what changes you will need to make for the best possible results.