A summary of guidelines for women's tops handmade by ModestAnytime.com, based on halacha.
The neckline of the blouse must cover all that is below the neck. The neck is vertical while the shoulders are horizontal. Therefore, the area to the right and left at the base of the neck that is more horizontal than vertical is technically the shoulder and must be fully covered. The collarbone at the front, the shoulders at the sides and the upper back at the rear must all be covered. See diagrams below.
At ModestAnytime.com, we try to adhere to these standards by providing tops with high necklines and reinforce some neckline styles with a mock neckband.
Sleeves covering the upper section of the arms should be semi-fitted (NOT tight, like wearing hosiery), and may not show the exact shape of the arms, because the "limbs of the upper arms" attract attention when fully displayed. The sleeves must be of a width and length that when the person lifts up her arms or carries a child, the sleeves will not ride up above the elbows. They must therefore extend to about 3 inches beyond the elbow and not be too wide. Many practice the commmendable Hiddur of wearing three-quarter sleeves or even full length sleeves.
At ModestAnytime.com, we try to adhere to these standards. Our tops are neither tight nor too wide as they are designed with ample ease at the sleeves. Our sleeve lengths are at least 3/4 but most of them are full length sleeves reaching the wrists (mehudar). We do not use lycra or spandex in the fabric and recommend that you do not do so either. Layering shells made with lycra/spandex/elastane that are sold at retail stores today (KikiRiki, Shel Sheli, Junees, etc.) are too tight-fitting at the body and sleeves and therefore, do not meet the minimal Jewish modesty standards.
Lycra/spandex/elastane, however has its place in neckbands, sleeve cuffs and headbands which reinforce the neckline, cuff opening and snoods.
Tops and blouses should be at least semi-fitted (kosher)around the upper part of the body or better,relaxed fit (mehadrin), to ensure that they do not display the shape of the area (chest). The garment does not, however, have to totally camouflage the features of the body, and as long as no more than "a general presence" is noticeable, it is Kosher.
At ModestAnytime.com, our tops are always made to be either semi-fitted or relaxed fit and are deemed Kosher or Mehadrin. We provide modest and refined alternatives to the most immodest Lycra/Spandex/Elastane tops you find at retail stores (Kiki Riki, Junees, Shel Sheli, etc.) that cling to your body. Read our article on "Modest Kosher Fit" to get an idea of the ease requirements.
Reference: "Modesty, an Adornment for Life", and "The Tznius Handbook" by Rabbi Eliyahu Falk