Hi, if your ordering them online, i would suggest to go into a shop and try them on just to see which size you are.
Quite a few shops over here do stock them (try the posh looking shoe shops!).
The ones i got were knee high ones, and im usually a size 5, but i got a size 6 just so it was a bit looser on the leg.
hope this helps.
The two pairs I've bought have always been true to size. They do feel a bit tight to start with, whilst the fleece is at its fluffiest, but soon loosen a bit.