Shoes on Savannahs are listed in European sizes, and a lot of the designer flats and pumps come in half sizes as well, so a UK shopper who is usually a 5 will want to check the size chart rather than guess. There's a Size chart link next to the size buttons on every product page. Jeans are sized by waist measurement in the same way the designers label them.
The product details panel under each item is worth opening before you buy. Bags list their measurements in centimetres, the strap length and the number of compartments, and many pieces say where they were made. With a short return window and a fee for sending things back, a minute spent on those details can save a return.
If you're in Stockholm, the boutique on Birger Jarlsgatan can be booked through the site for a private appointment, which is handy for trying shoes from labels that come up small or large.