It's a retailer with a bunch of different lines purchased from a variety of designers, plus their own in-house line. So yes, sizing and quality is going to vary by designer. The price range is wide for the same reason.
Modcloth itself is not responsible for the quality and sizing of everything, because they don't make it. Their customer service is quite good and returns are very easy. Chat assistance is available for all kinds of questions including sizing. They analyze each item and provide info if sizing seems off, and encourage reviews and photos from purchasers.
I do recommend. Read reviews, use chat and/or "Mod Stylist," and be willing to return or buy multiple sizes. Modcloth collects a wide variety of really unique items, including difficult to find international designers.
