Read the reviews
Most online shop allow their customers to rate and review the items they've purchased. This is extremely beneficial for shoppers, who can find out whether the colour and sizing of an item is the way the product description is portraying. But a note of warning here as well, some unscrupulous shops may add their own reviews as customers, if all the reviews are good, then there may be something wrong, unless it is a wonder product and is pleasing to everyone.
Choosing the right size
The height isn't usually a problem, but if you are extremely tall or short you will find that it is difficult getting the right size for you as most shops only sell heights of 52, 54 & 56 inches. Some Abaya shops will only sell a certain height and expect the customer to cut and sew to their own size after arrival, lets face it not everyone can sew or have the machinery to do so.
The worst thing is that many Islamic shops will sell one size in 'large' for all their Jilbabs or Abayas, this is to avoid stocking too many sizes. If you are extremely slim the idea of having something so baggy to wear is appalling. Islamic guidelines on women's dress is that it should be loose, but if it is too loose you may have problems wearing it comfortably.
Measure your existing Abayas
Most online Islamic shops selling Abayas or Jilbabs will have exact measurements of the garment in cm or inches. Measure your best fitting existing garments and compare. This information will allow you to choose the size correctly, one shops medium size may be another shops large size.