I buy nearly all my clothes online, as I really dislike clothes shopping! When I use a site for the first time it might take a few goes to work out what size fits me but then after that it's usually ok.
I've only sent one item back for being the wrong colour and that was my husband's fault - the dress was available in a dark blue or an orange, I had wanted the orange but he persuaded me to get the blue... when it arrived I just knew I would never wear it so sent it back and exchanged it for the orange, which I love and wear a LOT!
Most online sites here have free returns so it's no bother to send stuff back if it doesn't fit. I would far rather do that than have to spend a day at the shops!