If you are not sure what size you can wear, and feel that trying on everything is a hassle, or don't want to be disappointed not being able to wear something you'd like to try on.
Do you know the "back to back" method?
You don't need a fitting room, but you will need someone to help you, a friend or family.
Simply place the shirt or sweater that caught your eye, back to back to your back. Check if the shoulder matches your shoulder. If the shirt/sweater's shoulder is wider than yours, it's big, if it's narrower than yours, it's small.
Why not the front? Because your neck gets in the way and you can't get the correct alignment. Shoulder to shoulder across the back will give you a pretty good idea of the fit.
If you are worried about the fit around your breast, get a string or ribbon, wind it around your breast so that fits loosely, and cut off that length. Take the string to the store, fold it in half, and if the string is shorter than the width of the shirt/sweater at the breast, then you will be able to wear it. The closer the width is to the length of the string/ribbon, the better fit it is. However, for a fit comfortable enough for you to move in, you should add about 2-3 inches to the string/ribbon. That will give you an extra 5 or so inches around your chest. If you prefer a tighter fit, make the extra 2-3 inches around your chest. (Um, FemmeMode, correct me if I am too skimpy or generous with the extra inches to add.)
You can adopt this string/ribbon method for your shoulders, come to think of it. Have a friend or family measure your shoulder with the string/ribbon, and cut it at that length. Then, you can check the shoulders of the shirt/sweater at the stores armed with that string/ribbon.
Armed with the two strings/ribbons, you can check the stores, and you'll be able to get a fairly accurate idea of what will fit you.
Happy hunting and shopping, everyone!!